I'm going to be sexist for a moment and say that the ratio of men to women using handsfree devices in their cars is about 1000:1. Whenever I notice someone with a phone to an ear, it's almost always a woman, and when I see someone appearing to be talking on a handsfree, its always (not almost always) a man. Of course, that could mean more men are talking to themselves, but you can usually see the Borg-like earpiece. The only exception seems to be Nextel users (who are the most annoying cell-phone users because they have the speaker phones screeching at top volume, which they tend to match when talking back...plus that annoying bleep sound...ugh). Also, no woman I know uses a handsfree, and I know some guys who bought them for their wives, and they don't use them for various reasons.
I always use my bluetooth earpiece or speaker phone when I'm driving, so at least my cell phone use is no more distracting than a conversation with a passenger.
