I have had (and still have) a couple Garmins, a Tomtom, Eclipse GPS (on my head unit), and have the Chrysler RER nav (in the SRT).
Tomtom- If we would have known how the TomTom worked, we would have bought two (one to shit on and one to cover it up with)....nuff said there
Chrysler Nav (RER)- refer to TomTom
Garmin Streetpilot- Simple to use and update, the one I had had no fancy features like voice POI's, and streetnames, etc. But for what it was, it was great. Looked like a little CRT monitor, quite cute.
Garmin Nuvi- Not sure of the model, but it's a 5.2" screen I think (big sucker). Second favorite unit by far. Has voice POI and streetnames, and I have the lifetime maps, plus it's a thin LCD screen. Only downfall, no battery backup, so if you unplug it, it turns off, but it will save your route. Streetpilot would stay on...
Eclipse GPS- Flash based system that comes with my Eclipse AVN726E, updatable by SD card. Super easy to navigate, more POI's than any of my other units (by far). Voice everything, lane warnings, speed warnings, etc. And all the different voices you can imagine, I like the Dutch girl..
Favorite unit of all of my GPS.
I also have GPS on our iphones, but not very handy when I'm driving...