TomTom has been a little late to the text-to-speech party, with most competitors already offering this important function. Magellan has now jumped into the game with a very nice nav app as well, and at an introductory price that undercuts the TomTom app by twenty bucks.
All of the major navigation apps for the iPhone are quickly approaching feature parity. At this point, your main decision is whether you want the data streamed to your phone (as the AT&T app does), or have maps and data built into the app (like TomTom, Navigon, Magellan and some others).
Looming over whatever decision you make is trying to figure out if Apple will ever allow the Google turn-by-turn navigation app into the app store. It's going to be free, and Google says it wants the app to be on the iPhone. Of course things haven't been going swimmingly between Google and Apple of late (Google Voice, Android competition), so it's just a guess as to how that will all turn out.
If you need a navigation app in order to get to Grandma's house for the holidays, it's probably a good time to buy one. However, If you're willing to wait for the possibility of Google's free turn-by-turn nav app making it to the iPhone, you might be rewarded for your patience.