Not as bad as you would think. Parts aren't anywhere near as bad as the used to be. Sure, not as cheap as the Domestics or standard imports...but not that much more either. Plus, it seems as though you are buying new enough to have a warranty. So, that makes alot of the repair costs mute.
This is coming from a long history of BMW ownership. My last one was a 750il with the problem child V12. I had well over 250K on it before the trans went. All it ever took (besides the normal maintenance) was some suspension bushings. That car could move out too. My wife's last one, before her X5, was a 06 M5. Never had a single problem with that car and I beat the hell out of it. It was an amazing machine. Then a drunk driver came along.....
I have had a only one Benz, granted it was about 10 years ago and they have changed alot, but they just didn't feel as performance oriented as my BMW's have. I know the new ones are much different creatures. I will definitely need to try one out again.
Just my .02 Good Luck!