You Can't Topo Gaia GPS
I've used a lot of topo maps in my travels - National Geographic maps, Terraserver print-outs, OpenCycle Maps, and I even gave prints of Google Terrain maps a whirl. But, nothing compares to MyTopo maps.
That's why I'm glad my company is able to provide MyTopo iPhone topo maps. In fact, Gaia GPS (released just last week) is the only iPhone app to include MyTopo's maps of Canada and the United States. Gaia GPS also lets you create iPhone offline maps for when you don't have an internet connection.
Topo maps on the iPhone are even better, in my opinion, than print-outs, because it is invaluable to see yourself on the map, particularly if you are an imperfect navigator like me.