You're going to lose ASSIST.
You're going to lose your chimes.
You're going to lose any parking sensors.
If you care about any of those, I'd get the head unit recoded to Stereo and then use the speaker outputs to drive some speakers for that function. We used a center speaker just for chimes and front sensors and rear HiFi tweeters under the stock grilles just for rear parking alert.
The old head unit fits where the Sirius tuner is if you take the faceplate off. If you have Sirius you can take that out and sell it.
The Metra relocate kit is ugly and has a different color and finish than the Metra dash kit. You get better results with a pro refinishing it.
We just finished installing an Alpine nav in a 2010 with the stock head unit in the trunk, on the MOST and CAN buses and with chimes and front and rear parking sensors all retained, with steering-wheel controls, a flush USB in the console, the BMW AUX jack wired into the Alpine's AUX, and a backup camera. Works really well. We also installed our Level 2 audio upgrade so it sounds great, but I don't think it sounds any better than the Pro CD player would have until I use the parametric EQ and time correction settings.