The one major problem with gps apps in the app store, is the updates. With the appstore, you do not have incremental updates. If there is a new version of the app, the old one gets deleted and the new one installed. This works like a charm on apps that are a few kbs to a couple of megs, but gps apps, usually run into the hundreds of megs, and for instance with the US maps, it can run into gigs.
So everytime there is a map update, you will have to download the complete app, and boy that is a lot of download, if you are not on wifi, it becomes and expensive exercise.
But also, with saying that...I love the gps apps on my iphone, bought the navigon, and it works like a charm, and has a gorgeous interface, thinking of getting the tomtom too, but will wait till I have some odd $80 lying around to get it