I don't think that a dedicated car device is a must (the way that it might be in aviation).
I use a HP4700 PDA with TomTom5. TT5 actually runs properly in the 640x480 (TT3 didn't) display of this PDA. The result is excellent. TT is so good in fact that one could almost drive by the voice prompts alone.
The problem with a dedicated car device is that unless it is a part of the vehicle and not removable, it is certain to get nicked, and the window(s) will be smashed and/or doors damaged to get at it. (Don't I know it).
So one must remove it, and if one is going to carry it about then may as well get something that can be used for something else e.g. checking email, street nav, etc. I have a Audiovox RTM8000 PCMCIA GPRS card in my HP4700 and it works a treat.
If i was doing serious street-level navigation for living e.g. a self employed delivery driver then I would get a 8" tablet like the Motion ls800, with a car mount, and run a nav package on that. That would give a better overall picture.