I use my iPaq with a car mount Tom Tom kit and have good results. A friend uses the jacket version and again has good results. The only problem I've had is in changing cars. The actual sat receiver is powered from the cigarette lighter which on my old car had a constant feed. On the new one the cigarette lighter is only powered with the ignition on and it now takes 20 - 40 mins to get a decent signal. I don't think my car has windscreen shielding problems as well but I'm not sure.

Handnav.co.uk do decent car mounts (for phones as well) but a friend got the same unit as I did for chepaer from elsewhere so if you go that route do a search for brodit products as well.