This is the Sony CDX-GT550UI I will be installing.
Frustrated by the fact that I would have to go to the Audi dealer and pay them money to have them hook up diagnostic tools to my car to retrieve my stereo code (good one!), I decided the stock system could lick my bean bag, so I swapped in a Sony CDX-GT550UI. This stereo has been great in all the car's I've installed it (4 or 5 of them so far), seriously, when this head unit goes on sale I buy them 3 at a time. Anywho, most importantly, it has USB / AUX inputs on top of the usual CD/AM/FM options, and the USB interface is mint, I can plug my smart phone into it and listen to the music stored on the phone while at the same time charging, or I can just drop a USB loaded with music into it, and even throw an MP3 CD loaded to the gills and have some aural entertainment. I should also mention that this head unit's sound is incredible, it has some DM settings (dynamic something or other) and XPLOD sound settings that make even your crappy stock speakers sound just incredible, and all for under 100$.
I hesitated at first because the stock head unit is larger than DIN. I found out once I pulled the stock head unit out that the mounting was standard DIN which makes my life a lot easier, however the face plate was quite a bit wider, so I would end up with a gap.