I use Tomtom and Waze.
Tomtom is very good and needn't any 3G connection until you need traffic information.
Waze is better on some point, firstly it's a free GPS application. Map are updated regularly because each user can povide improvment to Waze Map. But in order to be fully functionnal, Waze need a 3G connection for map download and also traffic information.
Traffic information are reliable because they are filled by all users who use the tool.
Care about your iphone, if it's used for a too long time in GPS mode, it would shut down automatically, in fact iPhone prevent from temperature damage, and shut down itself if temperature is too hot.