OK, I thought someone else would jump in here, but I guess not. I run tablets in my cars. All are second generation running XP.
The Vision tablet above is one of the initial third generation tablets running XP Tablet edition and does require the special pen for writing on the screen. See
Wndows XP Tablet PC website for more details.
The two main reasons I run second generation are the docking stations and the passive digitizer. I do use them outside of the car so mobile dock is a must for me. The major drawbacks are color screens are not extremely bright in direct sunlight, palm rejection of digitizer means I have to poke the screen with my fingernail instead of my fingertip, slow processors (200-600MHz), and no stereo line-in on the latter, faster models.
If the pen issue does not deter you, I highly recommend you investigate the available docking options for the tablet as well as available accessories. For example internal WiFi is pretty much useless when mounted in-dash, some tablet USB ports are 1.1 not 2.0 and cannot support multiple daisy chains.