Hi!
Alturion seems to be a must especially if you have a GPRS connection and a lot of money.
I've been using Navigon (also not cheap), but now it's not available for PC anymore (unless it changed recently).
Right now I'm using Infomap Navigator Pro. (ver. 4.3.23) under Linux-KDE(Mandrake 10.2).
It does not have TMC or updated traffic info but it brings me where I want.
Previous versions had several bugs and where hardly usable. Now I can start to recommend it.
The price is really attractive, and the people respond more than less when you have some trouble.
It does not have a sexy 3D map view, but its CarPC mode and its highly configurable interface (defaults settings are good) make it suitable for a carputer.
I prefer Linux mainly because when I update my system I prefer to do it from scratch. And under windows I hardly remember how i did something, in which (really important) sequence etc...
In Linux (especially thanks to KDE) in one evening I can set up my carputer completely (including install a robust system, recopying music files, installing maps, pairing my bluetooth phone for playlist management etc...). Also EPIA Ms are well supported now under linux.
I live in France and I am going frequently in Germany via Austria and Switzerland and the only problems I had where related to map mistakes (TeleAtlas).

I'm stopping here!