I've got a Tom Tom Go. Absolutely fantastic. Portable as well, so I managed to rescue it from my burning Ferrari (risking life and limb ) and now use it in the 993.
I had a '96 Turbo4 and that had factory fitted Porsche head unit with Nav linked to a Nokia surround sound system. All worked very well.... it's the same head unit that was factory fitted to my 996 GT2, you can upgrade it with the standard Becker CD's