I am maintaining an annotated list of iPhone gps/mapping apps which you ought to look at to learn more about the options. There are 6 or 7 worthy choices (including Gaia GPS which has already been mentioned). There is no one app that does everything, and if you're serious about mapping apps then you'll want more than one. For use in the U.S. I regularly use four: Topo Maps, Gaia GPS, GPS Kit, and Maplets
And, to learn more about how to deal with battery drain and how to choose a gps/mapping app, you ought to read the Instructions for using iPhone as Backpacking GPS/Mapping device.
Since getting my iPhone, I have used it on a six week hike in Australia, a six week hike in Turkey, and many shorter multi-day hikes in the Bay Area. Other than mapping/gps, the iPhone apps that I use while backpacking:
Michael Morcombe eGuide to the Birds of Australia, Birds of Northern Europe. Sibley eGuide to Birds of North America.
Pano (a terrific little app for taking panoramas)
Read It Later (for saving wiki pages to read offline)
GoodReader (a nice viewer for a wide variety of document formats including doc, xls, pdf, etc)
CSV Touch (for lists of things I want to have with me)
DropBox (can't live without it, for sharing files between iPhone, desktop, laptop, husband's laptop)
Emerald Chronometer (for sun, moon, planet info; and because it's hands-down the most beautiful app I own)
Shralp tide (free easy tide charts)
RiverGuide for Kayakers (nice view of the US stream flows, requires connection, so useful for planning a trip, but not while in field)