I have been using the HD650s for 5 days now.
I am very pleased indeed. I wish I had got them sooner.

My previous headphones (an old Philips set) had quite a loudness curve, and I was always getting nasty surprises when listening back to mixes on other systems.

The HD650s are superb; definitely in a new league. Lovely for drums mixing, and quite flat in the mid-upper to top frequencies, so that there is no surprise clacking later on when it comes to playback on small speakers.

The bass end is quite honest, but still lively for those who like a bit more bass when using headphones. Haven't tried these out with any strings or brass yet, but am optimistic.

PROs
Great, alive sound. Like listening to some good speakers.
Honest.
Nice, flat bass response.
Wonderful build quality.
Long lead.
Comfortable.

CONs
A bit noisy outside - i.e. not great if you are using them when silence is needed (baby sleeping in the same room?).
Let's a lot of sound in from outside - this is a pro for me, but probably a con for others.

Summary - I would definitely recommend these to my colleagues and friends.

Fantastic headphones, I love checking mixes on them.

They are very comfortable and sound fantastic.

Not good for monitoring when recording as they're open back so no isolation.

I use the HD280 Pro for recording and they are great ! perfect for isolation.

650 and 600 are both great reference cans. Adam Ayan from Gateway Mastering suggested me using these a year and a half ago and now I am happy to carry them with me as a reference check.

600 are overall balanced, modest stereo (it goes wide when it's wide). Open top.

650 are a bit more high-mid intelligible but at costs of dynamic lows. For me they are too bright, but more detailed.

Both are comfortable for long work without being fatigued.

Make sure you have a good amp to drive them - they have a pretty high impedance (300 Ohm). I prefer Grace Design.

I made my way to the HD650 starting with the HD555, then HD600, then HD650. I also own other headphones ranging from entry-level to high-end. I'll list them so you understand the context in which I judge the HD650.

My other headphones are:
Audez'e LCD-2
Stax 007MK2 (with Stax SRM-717 amp)
Denon AH-D950
Denon AH-D7000
Sennheiser HD600
Sennheiser HD555
Audio-Technica ATH-M50
Equation RP-21
Pioneer SE-DJ5000
Westone 3
Westone 4
Shure SE-530PTH
Shure SE-535
Shure E4C
and my studio monitors are Klein+Hummel O300D's.

The HD650 is one of the most neutral sounding headphones I've heard, and it's quite comfortable to wear, with velour earcups that don't get sweaty (unlike leather/pleather), comfortable clamping force (I have a pretty big head), and fairly light weight. The cables are also light weight and don't drag.

There are no frequencies that are hyped, and while the upper bass has a gentle hump, it isn't detrimental at all. The mids and treble are liquid and natural, and provide ample detail without being fatiguing, too lush, or too bright. The only thing the HD650 lacks is an authoritative sub-bass. It just doesn't deliver lower sub-bass at adequate level when compared to other headphones that do extend deeper and are more authoritative (such as the LCD-2, 007mk2, M50). You can try to boost the lower sub-bass with EQ to compensate if it bothers you.

I don't normally use the HD650 since I have better headphones (LCD-2 is my main headphone), but for checking the mids and treble, the HD650 is very helpful. For it's price range, it's one of the best sounding headphones out there.

Pros:
-Mostly neutral/balanced sounding (with exception to lower sub-bass)
-Non-fatiguing sonic signature
-Comfortable to wear
-Durable
-Velour earcups prevent sweaty ears

Cons:
-Lack authoritative lower sub-bass compared to some headphones

Really love the sound of the HD650's...warm, smooth, open. Great quality audiophile headphones.

The only problem is they can be a little too flattering sometimes, they have a habit of making almost everything sound good, which isn't always what's required! Also, make sure to replace the (extremely comfortable!) ear pads once in a while, as they got worn the headphones sit closer to your ear and enhance the lower mids a little. High end detail could be a little better, removing the thin foam from the inside helps a bit. All in all an amazing set of phones for listening on, but perhaps not as revealing as some might hope for in demanding situations....fine for checking clicks, pops or unwanted artefacts though (makes the experience more enjoyable actually!) and great for impressing clients! heh

Have had these headphones for about 3 years now and am still very much impressed with their sound. These work perfectly to reference your mixes on as well as for your own listening pleasure.

They reveal quite a lot of detail, are very clear, open, crisp and their sound stage is really nice compared to other headphones. For that reason they are suitable to listen back how your mix translates on similar mediums.
The HD650's are, as mentioned before, not completely neutral sounding. They do tend to make things a bit sweeter.

Apart from the quality of sound I am also charmed by their build quality. They are very sturdy, yet incredibly comfortable. I can wear them for hours on end without actually feeling like I have headphones on. The foam bits sit snug around your ears.
Another good thing to know is that the cable can be replaced easily by yourself. So no hassle if the cable wears out. So far that hasn't been a problem as after all this time I haven't had to replace a single thing and I wear them on a daily basis.

I would advise a headphone amplifier to drive these even though I do not have one at my disposal at the moment. That way you can be sure you will get the best out of them.

All in all this is a purchase which will last you several years, a definite great investment!

Of all the open-back headphones I have owned and used I can honestly say that the HD650s offer best value and stunning sound for the price. You would have to shell out several times the money for something that sounds better.

I first bought a pair of HD650s over 3 years ago. I was looking for a decent pair of headphone for night-time mixing at home... Driven by the headphone amp on my RME Fireface 800, I was surprised just how revealing these cans were. When listening to previous mixes and music on my iTunes to get used to the HD650s, I noticed elements that I hadn't heard before. Also, any new mixes I did translated better when I used a combination of my monitors and the HD650s.

The HD650s have a nice smooth response with extended bottom-end to the HD600s. I find even listening to my music collection quite a joy on these cans. I use them almost every day for night-time music making and mixing in my bedroom and these are some of the most comfortable can to wear. They are light and the sound is non-fatiguing and open. It's almost like not wearing any headphones because you can hear everything in the room. Being open-back design these are NOT for tracking because there is significant leak or sound.

Sennheiser support is also fantastic.. The left channel had a bit of distortion after 2.5 years of use and they repaired it free of charge. Most parts are replaceable and easy to find online or through Sennheiser. Can't recommend these headphones enough and they are great value!

This wil be an odd review, but hey, let's have some fun. I'm gad that there are other reviews here from long term users her because most professional reviews take place right out of the box. However, headphones and monitors obviously need time to burn in . The biggest complaint of the 650's is the darkness and the 3-blobs-in the-head syndrome. Nicely there is no mention of either of them in the review move yet they plague pro reviews all over the net.

The 3-blogs syndrome is when you hear very overly-obvious center and stereo images. But the "northwest and northeast" are subdued giving the listener the feeling that there are three blobs of sound- one left, one center, one right.

When I first put on the 650's, these two issues were so obvious, I nearly sent them back. Even despite the fact that I had only listened for a few hours, it was so far from what I would consider acceptable for mastering, I nearly called it quits. My audio-god at sweetwater told me to keep burning them in and give it some time. Things have begun to change wildly so I thought I'd share my progress.

First, I'm 90+ hours into a planned 200 hour burn-in. The blobs are disappearing. Yet the phones remain a little dark and I assume from many read reviews that they will remain to some event dark. But this is not because of lack of highs or enhanced mid bass. Its actually a small lack of lower treble. That said, so far the clarity is amazing, it's somewhat humorous. The sound is silk, accurate, even, and for a mastering engineer who will be using these less for tonal adjustments and more for surgical monitoring, I have only listened to a few phones that provide this depth of clarity. WOW!

Here we are so far:

Feb 5, 2012 evening 5 hrs WHOA DARK!!

Feb 5, 2012 overnight 8.5 noticing amazing detail. Very dark with unclear low sibilance

Feb 6, 2012 evening 2

Feb 6, 2012 overnight 6 getting worried. despite clarity, its so dull in the low high range/mid mid range, I'm not sure this is a break in thing

Feb 8, 2012 overnight 8.5 Karl, my gear god says give full break in period so 20 down 180 to go : (

Feb 9, 2012 night 5 oddly only a few hours later, I'm starting to hear some changes. a little less dull, better clarity in the low sibilance.

Feb 9, 2012 overnight 8 head image opening up a bit

Feb 10, 2012overnight 12.5 hearing a lot of distortion from tunes I’m familiar with, man, these phones might make the loudness wars even uglier!

Feb 13, 2012overnight 13

Feb 20, 2012overnight 11

Feb 21, 2012evening 3.5 head image significantly improved at this point, a little less dull but still lacking a touch of clarity in the low treble area. 80 down, 120 to go

Feb 21, 2012overnight 9.5

My scores are not final. 92.5 hours in. Keep you posted ; )

Unless you spend a hell of a lot more cash, the Sennheiser HD650's are pretty much the best you are going to get.

By way of comparison, I have also owned and used all these closed back headphones HD250, DT770 PRO, DT250, DT100, DT150, SRH840, M1000 and ATH-M50 (the new studio industry standard). Overall the Sony MDR-7509 are better than all of them with the ATH-M50’s coming a close second, along with surprise gems Studiospares M1000. Of course the 650's are open backed so it puts them in a different league.

Quite simply put, they are very flat in frequency response. A lot more flat than any of the above models. No hyping anywhere in the 2KHz and upwards range to the top. To point out any areas of weakness would be there's a very slight hump from about 150Hz to about 280Hz and a pretty sharp dropping off from about 50Hz down (compared to MTH-M50's for example). Soundstaging blows the others above out the water. They just sound lovely to listen to good mixes on - both for mixing/mastering and general music listening. Anything not good will sound crap - it shows up over clipped masters like nobodies business. I have done a lot of mixing on these and mixes always translate really well to other systems, with the exception of sometimes you will have a little too much energy in the sub regions because of the dropping off on the headphones. This is very slight though, a dB or 2.

They are very well made and comfortable, if a little tight on me. You get what you pay for basically. Would love to try the 3 times more expensive HD800's, although some reports say they are a little too bright 8K and up, which you can see on the frequency plots.

Buy with absolute confidence if you want to hear what your music actually sounds like.

I've seen on YouTube that some people use HD650 for mixing and critical listening, so I decided to write this review from a mix engineer point of view, not from an audiophile point of view, please be aware.

The best things about these headphones is that they are extremely comfortable and not fatiguing at all! I could listen to them for hours without any signs of fatigue and when I took them off there was no need to readjust to my monitor speakers (which happens when headphones are overly bright or too bass-heavy), meaning that HD650 sound is neutral. This is a great advantage when it comes to long listening sessions.

On the downside, I often could not hear some changes that I was doing in EQ or compression, and literally everything I listened on them sounded nice. That's why I think that for mixing and critical listening purposes these headphones are not the best choice. They are audiophile headphones and not as revealing as, let's say, Beyerdynamic DT 990. That's why, if you have mixing in mind, I'd recommend the latter. Besides, they're more than 2 times cheaper than Sennheiser’s (at the time of writing this).

To sum up. if you’re after listening music for pleasure for long sessions, then HD650 is a great choice. If your primary objective is sound mixing and critical listening, then I'd go for something like Beyerdynamic DT 990. Plus, a decent pair of monitors, of course. Please don't mix on headphones alone, it will most likely not translate well to other sources.

Natural and transparent headphonez. I prefer the HD600. The high Frequencies no the 600 are slightly more natural compared to the 650, which has a bit to much on the top. This is fantastic to work on..