One of the best features of the iPhone app I have been working on is the topo maps we include. Gaia GPS is the only app to provide these iPhone topo maps, and they are beyond compare.
You can get lower resolution topo maps via other apps, and even the high-res USGS maps from the Topo Maps application, but none of these maps have the same detail that Gaia GPS provides.
Gaia is also cool because it lets you do iPhone offline maps, for when you don’t have an internet connection.