I own the Symphony (mk1) in my recording studio, and I really wanted to step up my game in my woodwind teaching studio. Because the quality of the Symphony is so high, I really felt that sticking with Apogee, especially at this price point, was a no brainer. I also decided to include the optional remote with my order. Both items were a good decision.
I will review a few key points here, but suffice it to say that I feel that Apogee made wise decisions on the quality and features for the price point they were targeting.

Converters:
Nice. I have not AB'd them with my Symphony or Mytek 192, but they are a HUGE improvement over my little Mackie Blackjack, as well they should be at this price and recent design. Much better sound quality in every respect.

Inputs/Outputs:
Exactly what you need in a 2 channel interface.

Headphone out:
More than adequate for most situations, however, in my teaching studio, I use phones a lot, and I have learned that a lot goes into powering headphones, so I ordered the Neve headphone amp and Sennheiser HD650's, and ran the 1/4" headphone out of the Element into the 1/8" input on the Neve. Immediately, the low end and transient response of the headphones came alive, in comparison to driving the phones directly from the Element. Not a knock on the Apogee, but just what happens when the headphone amp costs nearly as much as the interface.

Mic Pre's:
Huge improvement over the Mackie. And I'm confident that the quality of the mic pre's and just about everything on the unit is greatly increased by not making this thing bus powered. But, again, I'm pretty spoiled from the recording studio, so I got 2 Manley Core's and bypassed the pre's in the Element. Great addition, great improvement. But are the factory pre's in the Element of good quality, and can you make great sounding records with them? Absolutely.

Software:
Excellent. Killer. Love that you can change the input pad on the Element right there in the software, so if you're using separate pre's, just a mouse click on the +4 db and the XLR input is ready for action.

Remote Control:
This thing rocks. And at $199, it's a bargain. The Element software makes customizing the buttons a snap!
CAUTION: I couldn't find the included cable to connect the remote. After searching for about 20 minutes, I finally realized that there is a false bottom inside the box. There it was!

Build quality (of the Element and the remote):
Absolutely superb.

Apogee customer support:
Wow--I'm such a fan of the "Chat" button on the front page of their website. Links you straight to Tech Support, and response is very quick. Absolutely brilliant!!! Just that button is reason enough to buy Apogee.

I'd like to say again that adding a Neve Headphone amp and Manley Channel Strips are in no way a knock on the quality of the Element 24. Apogee has delivered more than you have a right to expect in a $595 2 channel interface. In order to create a significantly better box, they would need to at least quadripal the asking price. The Element 24 is at the heart of my teaching studio, and it performs admirably. Highly recommended!

I was torn between buying an Element 24 and a UAD Apollo Twin MkII Duo. First, I contacted each company asking them why I should by theirs as opposed to the other. Apogee responded through their Chat Support within 10 seconds, and gave me detailed info on their Hardware Specs. UAD took 3 days to get back to me (email), and they were only trying to sell me on their plugins. When I asked for Specs, the UAD Pre-Sales/Tech Supporter got defensive while offering me NO details about their preamps, DAC's, or Headphone Pre's; again telling me that their plugins are the "best in the world", and again tried to sell me on their plugins... Apogee 1, UAD 0 (so far).
So, I went to my local Music Superstore, and they had an Apollo 8 hooked-up. I asked for an electric guitar, and started testing the Apollo 8, which is a far more powerful version than their Twins. I loaded-up the "Engl" Guitar Amp plugin, and great! I was rockin! So I tried to add some Reverb...and coudn't because the Apollo's DSP was Maxed-Out after the 1 Guitar Plugin!!! Hmmpphhh! (UAD Minus-1!) Not impressed! (Again, I was using a unit with MORE DSP than the Twin!). As for sound quality, nothing stood out. It was flat, even with the plugin.
So I went with the Apogee Element 24. Blindly (I can return it if I want). I got home, installed everything within 10 minutes, and WOW! The sound is incredible. The detail and sound quality is like nothing I've ever heard. First, I did the Headphone test, and I was hearing things I never had before: Shimmering reverb tails, panning that was fluid, percussive sounds have a great crunchy yet clear sound. Then next I hooked it up to my cheap, KRK Rokit5's. The DAC's provided such an amazing sound to such crappy speakers with their technology to deliver Output scaled to the Speakers response. Bass is solid and tight, Highs that are crisp and detailed. Playing some virtual instruments like Massive or Battery, Presets were jumping out with new life and energy, and detail. Listening to songs in iTunes was like hearing ReMastered versions - again I was hearing detail I had never heard before.
The Element Control Software is daunting at first, but with another quick Chat with Apogee tech support, I had Logic running into the Element hardware, which I could mix separately in the Element Software, and Track in real time WITH FX like reverb and Guitar Rig running on AUX Buses, again processed through the Element hardware. I could then Print my recording using the FX Send Outputs back into Logic, all the while the Element's DSP handled everything in pretty much zero latency. So even recording into Logic X, with "Direct Monitoring" enabled in Logic's Channel Strips, all sounds - live Guitar or Virtual Instruments, were fed into the Element's Hardware DSP for Gain-Stage matching, and I was amazed at the improved sound quality.
When they come out with their DualPath FX Plugins later this month, things will only get better (I can't see how they can improve on their sound, but tapping into the Element's DSP functionality is going to throw UAD's "Unison" Tech a good curveball).
I also saved $500 over the Apollo, which I plan on using to put towards better Monitors.
IMO, UAD is a plugin company that makes Audio Interfaces. Apogee is an Interface company that is coming out with plugins. I'd rather have my Horse before the Cart....
Summary: to utilize more than 1 or 2 of UAD's "best in the world" plugins on their Interfaces you'll need to upgrade it's DSP, probably a few times, making the price a couple thousand dollars. If money isn't an issue, well, have fun! With the Element, I was tracking in real time with many plugins (not UAD, but FabFilter, Waves, and N.I.), and it's only going to get better as Apogee's Engineers perfect their own version of "Unison Technology", but with far better Hardware specs that won't leave you needing to spend hundreds on UAD Plugins to achieve a desired sound - with the Element, their Hardware sounds great on its own without any plugins to depend on.

I find people get too hung up on sounds quality when the fact is that there is very little audible difference (to your audience) between even mid priced and top end convertors.

There's also way too much focus on the audio interface.
Its a process...it doesn't perform, it doesn't create, its a "waiter that brings food to your table". People that fuss over interfaces are the types that will forego bad food in a restaurant if the premises and service is good. If that's what you want then go to a lounge club...and save your food money.

Your music is gonna end up as a humble low grade mp3 so what you need from your interface is a set and forget do a good job and get out of the way unit which is reliable and easy to reconfigure so as not to disrupt workflow.
Any extras should help free up vital brainpower and creative juices or enhance performance.

And so that brings me to the Apogee element.
I've owned a few interfaces from the high end (at the time) creamware card (I hated it: any quality gains were outweighed by frustration in configuring it to the point where I didn't feel like making music), M-Audio (unreliable ASIO drivers), Mackie onyx (great but big), TC Konnekt (great but not enough outs for my talkbox setup), Komplete 6 (OK but fiddly).
RME and Apogees were always an aspiration my bedroom budget couldn't afford (lets skip the duet, which was focusrite quality but overpriced), until Apogee launched the element range.

The concept is genius, take the guts your top-line equipment, put it in a cheaper box and convert all the expensive control hardware into low cost software.
If you purchased a Quartet, Ensemble or Symphony you'd be kicking yourself....all the extra money just for a bit more tactile operation and sexier looking box, without remote control from your phone. Yes the element can be remotely controlled from your phone! And its the only interface that integrates seamlessly with Logic (big workflow advantage eg you can trim interface audio levels on dedicated apogee pots on each logic mixer channel). Furthermore it has mic talkback function that can use your computer mic and even your phone (software update coming to enable phone talkback). And of course the quality is great (look for blind tests on gearslutz if you don't believe the hype around its preamps and convertors...not that that is a major advantage, no1 hits have been made on far worse).

If you are a logic user, the element is a no brainer. If you don't then its still a no brainer because it does RME level performance but with more functionality (talkback, remote) with a more modern and easier to use control system.
For my money the only interfaces that come close are Focusrite and UAD but only because they offer onboard plugins, which offer workflow advantages when setting up cue mixes to inspire artists to perform better. But if you don't need onboard plugins then Element wins again (note you can use DAW plugins and software monitoring to achieve the same).

My only gripes are

• not bus powered. No issue if you aren't on the move but I lost the power supply during a move and its been a pain to get another. I hope this is something they address on the next iteration.
• Talkback function via remote control phone is tantalisingly possible but not software enabled yet.
• I always think these companies are missing a trick with their software control centres that don't employ preset templates for common setups. Apogee is not alone. Would be a useful update eg to be able to switch from tracking with multiple cue mixes to mixdown scenarios.

Its rare I eulogise about a bit of kit, but when people get design right it is worth shouting about. Good design strips out things you don't need, helps you get the job done better and gets out of the way. This unit is worthy of such praise because it set a bar that the others are now copying.