I've got the Nokia bluetooth kit and haven't had any problems at all, its a great piece of kit.
The only things it lacks are a method for charging the phone (which I've never missed at all) and there's no external aerial which would be handy to improve signal.
I believe there's is no way to connect it to the steering wheel controls (or to the car speakers in the E-Class) but overall I'm really pleased with it, just connects automatically to the phone in your pocket.
The parrot has a voice activated system and is hidden in the dash. It's also cheaper than the Nokia and connects to the car stereo. The holy grail would be steering wheel control, though I doubt this unit is capable. I'm fitting out the audio in my car and want to do the whole insatllation in one go.
Just can't find anyone who actually owns one.
I am a great fan of Nokia and its products amd am thinking of getting the bluetooth kit, the only downside that i have heard of is that it only works with nokia phones where as the sony version works with all bluetooth phones.
The Nokia kit does work with some Ericsson phones, I'm using mine with the T610 with no probs. The problem is that Nokia used the Handsfree bluetooth profile and Ericsson used the Headset profile, but newer phones like the T610 are able to use either. The only downside is that you're unable to change the carkit ringing tone (HELLO!!!! ) if you don't use a Nokia