A couple months ago I took shipment of the 3rd generation Mbox, which debuted back in 2010. Previously I had been using an Mbox 2 Pro. The new one is a complete revamp, and I'll run down a few of the things I'd like about it here.
Construction. The Mbox 2 Pro felt every bit the creaky piece of plastic that it was, which was always disappointing considering its price tag of several hundred dollars. Fortunately, the Mbox 3 is exactly the opposite. The dark metal casing is super sturdy, so it doesn't feel like my fingers will punch through when handling it.
Buttons and Knobs. The channel gain knobs are now a traditional round shape, with rubber grips, as opposed to the awkwardly thin ones of the previous model. A dim button takes the output down by a good 20 dB or so (I haven't measured), although it's fairly recessed and hard to push. It's usually easier to grab the large master volume knob. Far more interesting is the mono button, which sums the left and right channels. Previously I would have to do this within Pro Tools, and while that was of course workable, it was also cumbersome. Having a physical button to push is great for quick checks.
Software Mixer. Very straightforward and easy to use. The built-in reverb and delay effects are surprisingly nice to have when recording vocals.
There are a couple of things I'm not in love with on this model compared to the Mbox 2 Pro. Inputs and outputs are pared down, although this has more to do with the fact that it's not the "Pro" model than anything else. A second headphone input with its own volume adjustment would be really helpful for tracking someone other than myself. And the multi-function button, while seemingly useful, can only be mapped to start/stop record, undo, save session, and other things that are more easily achieved with a keyboard. I can't help but think that it could have been used better.
Still, if you happen to be in the market for a mid-level recording interface, the 3rd generation Mbox is a nice choice. Well worth the money.