I know you aren't asking about factory kits, but I should mention my iPhone 4 paired just fine with Uconnect in my old Chrysler and paired just fine with bluetooth in my new GMC SUV. I've heard Sync works as well but I haven't tried it. For aftermarket, take your time and look for something that doesn't require you to fumble with buttons to take a call. That defeats the purpose. With my factory (GM) kit, to take a call I can hit a button on the steering wheel or accept the call on the phone. Either works fine. If I get in the car on the phone, the call transfers to the car automatically after a few minutes. iPhone4 offers excellent control for car kits. Right there on the touchscreen there are options to transfer the call back to the phone or even use the iPhone as a speakerphone. Before my iPhone, I had a Blackberry that made controlling bluetooth seem like rocket science and definitely not something I would ever try to tweak in a moving car. On my iPhone, it's right there on the touchscreen. How nice.
Of course one down side to a bluetooth kit is it doesn't charge the phone. But I find my phone holds a charge fairly well so I don't mind. I can always add a car charger and I think my bluetooth kit would still work the same way.