I use a bluetooth connection to my Treo650 and the speed is acceptable. It's not super-fast by any means.
I had three downloads going at once and each download was pulling in as fast as a top-notch dialup connection. For in-car use, that's more than sufficient, IMHO.
But no, it's not free. I have two unlimited data plans: one for my Treo and another for my girlfriends' Blackberry. I don't care, as my boss pays for it.