My Jensen unit stopped working right, so I downloaded CoPilot on my iPhone. It's free (with a paid option if you want stuff like traffic). It works fairly well - my only complaint is with the way it handles highway exits. My old unit would tell me which lane to be in if the highway split, this one doesn't. It took me a while to figure out where it wanted me to go. It's also a little bit frustrating if there are two turns in quick succession: sometimes it will say "turn left, then turn right". But sometimes it just gives you the first direction, and then as soon as you turn it says "now turn right". Really frustrating if you've just turned left into the left lane of a big road and suddenly you need to get all the way to the right lane to go right with no warning...
All in all, though, for a free GPS app, it's great. It lets me listen to my music while using it, it's accurate, recalculates quickly if you make a wrong turn, and speaks street names (and quite intelligently). I was going to buy the TomTom or Navigon app, but I'm not sure what they can offer that this one doesn't...