I have used Garmin, Navigon, Igo, TomTom and Sygic.
Garmin was't really userfriendly in my opinion and I stopped testing it very quickly.
Igo is really nice navigation program, It is customizable, it has tmc, roadassistance. Programs starts not fast but when it starts it works fluently. There were two reasons I stopped using Igo: 1. Speech is really a chaos. Its not accurate, it replays a lot and the beep tone before the speech is really annoying. 2. GPS wasnt very accurate.
Sygic is almost like TomTom, but sometimes the calculating of the roads wasnt really logical for me, but its really nice alternative.
Navigon is the most extencive navigation program for PDA. It has visual road assistance for example. But it doesnt work very properly yet. I cant search for POI´s and its really heavy program. Speech is very good, pitty that it doesnt have querty keyboard.
I have used TomTom for a long time and looked every time to another, because it doest have any extras (it doesnt have road line assistance for example), but because its light, stable, userfriendly and everything works like it has to, I always went back to TomTom and now it is the only navigation program I use.