I've tried 5 programs, and Gaia GPS is my favorite. This is because Gaia has seamless USGS topos with a shaded-relief overlay, which I find very helpful. It also has a nice screen that shows a customizable trip computer, including total elevation gain in addition to altitude. Finally, it has an easy system for downloading large clusters of topos.
But there are other map programs that have higher resolution scans. And GPS Kit certainly seems to have the cleanest UI. Does anyone know if GPS kit also has USGS maps as an option?
With any program, though, you don't want to use tracking unless absolutely need to; but if you have a full topo map, why do you need tracking if you can always see where you are on the map?
I went on a 4-day backpacking trip with an iPhone 4 this past summer. I removed the sim card (this is the only way to turn off the cellular antenna but keep the GPS on). I checked Gaia GPS 10-12 times a day for 30 seconds to a minute, took a few minutes of videos, listened to about 6 hours of audiobooks, and used the LED as a flashlight for about a minute. After the trip I still had 79% battery life. And I found acquisition times as well as accuracy to be on par with Garmin H high-sensitivity receiver. The GPS in the iPhone 4 is MUCH better than previous models; I would only take the iPhone 4 out in the field.