Read a few posts with people doing that. I was nervous that the 12v converter would get too hot and the tablet itself.
I use the car each day, so little chance of the battery draining. Takes about 5seconds to power on, enable GPS, lock to satellites and load Sygic satnav. Happy with that
I also have this with an OBD2 unit. Looks great
http://www.appszoom.com/android_applications/tools/acarputer-gps-obd_clwfg.html
Well it's really easy, you can take a feed from the passenger footwell cig lighter, a decent 12v adapter shouldn't get too hot either, but like you say no big deal