Personally I have iGuidance 2009, Google Maps, Bing, and Waze.
Waze is my favorite in concept but still in beta and it shows. On the other hand it has voice turn-by-turn and real time traffic. I have iGuidance because I don't have nationwide data and have to travel outside my coverage area often enough to need it. Bing I like for finding stuff around me locally and the voice search although I don't get why it can't use the GPS as the current location as the default upon start and update that periodically. Google is really my least favorite of the bunch but I keep it around in case the others are not giving me what I want.
If Google would add voice turn-by-turn and a few other features from the Android version my opinion of it could turn around real quick.