I have had the MBox3Pro for a year now, long enough to share a few of my observations.
First of all, it is very heavy for its size. I kind of like that as it gives the unit a nice, solid feel, but it is something to consider if you plan to move it around on a regular basis. It is portable, but you're going to feel that weight in your backpack.
Let me relay the negatives first: at first I had trouble with the headphone outs. Half the time they wouldn't work on startup, requiring a reboot (or two), to activate them. A few times they would produce a loud hissing noise, requiring another restart. Very annoying, although powering off and on again (once or multiple times) would always fix the problem.
This is a known issue, and Avid has been working on a fix
What's more, I only got the included direct-monitoring reverb to work once, it has been MIA since. That is actually a good thing because the one time it DID work it sounded awful (and mono).
The good news is that a driver update seems to have fixed the headphone issues, I haven't had any problems with them since, so fingers crossed. I am a bit mystified as to how this problem was solved, because even after the driver update I continued to experience these problems for a while. These days, however, everything is great. Maybe I'll get the reverb to work properly (and in stereo) with a future update, too.
On to the positives: it sounds good (disclaimer: I do not have a $10,000 signal chain to validate this statement) and once in operation it just gets out of your way and on with the job. I really like the GUI of the mixer and it is flexible enough for my modest needs at home. You can get separate cue and headphone mixes on all outputs, or use them in parallel.
It is also noticeably more efficient than my Saffire LE. I'm primarily a Cubase/Nuendo user, and on my old iMac I would regularly have to freeze VI's to keep the system running. With the MB3P the same projects could run without freeze. I have since moved on to a quadcore MacPro and I hardly ever run out of room with the buffer set to 128 samples.
The mic pres do the job well enough, although you'll never mistake them for Neves. The softlimit function is handy, but it seriously chokes the sound when pushed, so you'd be advised to adjust your gains carefully. There is plenty of gain, btw, for both my mic and keyboards.
The mono button is a real selling point, but I find myself not using it as much as I thought I would, and when I do I find the increase in loudness irritating. This is obviously not a design flaw, but it would be nice to be able to attenuate the summed mono signal to levels more proportionate to its stereo counterpart.
All in all I like the Mbox3Pro, but you'd better make sure it fits your needs. Compared to similarly priced units from the competition, it has limited connectivity and zero expandability. I also dislike the combi breakout cable for SPDIF/wordclockIO/MIDI and the external PSU. Not something to complain about at this pricepoint ($629 +tax, in my case), though.
One last gripe, Avid support requires a support ticket, and one is issued when you register your product online. When I wanted to contact them about the headphone problems I found out my ticket had expired (I think it is like 60 or 90 days after registration) and I should purchase a new one for $40.
Of course, I didn't. Paying $40 to get someone on the phone because your product has a design flaw? I don't think so. So even if I'm generally happy and satisfied with my Mbox3Pro, I shan't be buying from Avid again anytime soon.
original review here