My problem with my t-mobile galaxy s2 is that 50 percent of the time it won't auto connect to my factory stereo on my 2004 Acura TL. I'm certain it's not the stereo. My Nexus One worked flawlessly with it.
What's really frustrating, is when it doesn't auto connect, I usually can't force the connection. In fact, when I try to turn bluetooth off and back on, it won't turn back on! I have to reboot the phone to get everything to work... Errrr!!! All this messing around with the phone while I'm driving defeats one of the main reasons for bluetooth... safety.
I really hate this. Everything else on my phone works great, but having a good bluetooth connection in my car is probably the most important feature on a phone that I need to work.