No carputer for me yet, but I have driven around on roadtrips with my dad's laptop for about 8 hours with a decent wireless card, no extra antenna. When actually driving (in decent sized cities), you tend to hit a point, and almost instantly get out of it. 802.11x generally didn't do well while we were moving. Usually it was hard to figure out where the spot was. The card's drivers (or maybe it's XP, dunno) had an Icon that popped up and turned green whenever there was a connection. But sometimes IE lagged behind on some stuff (using cached pages instead of updating while you were in the spot) and so it generally didn't work well.
Now for areas very densely populated, it might be a little easier, but for me, it didn't work to well.