When I use a map on the iPhone I've been using the google maps app (I haven't used Apple's maps for ages but having a quick look, it's showing me some useful info and nicely laid out too). The following link is several months old but makes some interesting reading about the Google Apple maps debate:
I still prefer 'Nokia Here' for driving directions over Google's navigation apps- it gives me more useful directions! (I've just found an iOS app 'Many maps' which gives me Google, Nokia Here, Bing & OpenTouch maps & navigation on my iPhone- it's interesting seeing them side by side).
(Update: ironically a friend who has had an iPhone for ages was convinced she was using Google maps. In fact she was using Apple maps -she didn't have google maps installed). A bit more brand awareness of Google maps!