I've had my 645ci for two months now. It is the most fun car I've ever driven. It's a head turner and eye catcher and a road grabber. BMW hit it out of the park with this one. Handling and performance are supurb you feel safe and solid in every turn. I like the lines on the 645CI better than the convertable which looks a bit choppy. After a month I got used to the improved I-Drive. The issues I've had are waiting for over two months for sirius radio and my new blackberry phone does not work with the bluetooth feature on the 645. This car is amazing, I look forward to seeing you in my rearview mirror.
Handling, pick up and responsiveness are the core of this beast but my favorite feature has to be the lines, this car is nothing short of stunning.
Make Sirius Radio a feature instead of an option, get the electronic interface issues fixed, ie. bluetooth and IPOD factory adapter.