I've been using Garmin GPS for years, most recently the 60 series.

This weekend I downloaded an iPhone app (MotionX GPS) on an iPhone 4S. I took them out side by side for a long hike this weekend and I was very impressed with the app. I was hiking in the East Bay hills in the SF Bay Area.

First the iPhone display is light years ahead of the Garmin in terms of quality and detail. The base maps were also superior in terms of the details they offered. I purchased the Garmin Topo for Western US 24K. To use Motion X when you're out of cell phone range, you have to download area maps, but this was quick and easy to do. You go to a map and draw a circle of the area you want to download. I downloaded about 50 square miles of maps in 10 minutes via wi-fi.

Both were about the same in terms of speed in acquiring satellites and maintaining contact even canyons with tree cover. The claimed "accuracy" was about the same, but Garmin more often claimed accuracy under 20 feet, when the Motion X would be claiming 30 feet or so. At the end of the hike the Garmin said I'd covered 8.42 miles, MotionX said I'd covered 8.38. I think that variance is within the tracking errors of each. Also one time in cover the Garmin said it could not acquire a signal while the MotionX claimed it had a signal. This was very surprising to me, given the 60s reputation.

MotionX did worse at speeds, it said my high speed was 22 miles per hour, and I don't go that fast even falling down a hill. This caused my average moving speed to be a bit higher on the MotionX (2.7 vs. 2.6mph) This could also be a function of the shorter distance measurement on the MotionX.

The only downsides I can see to the iPhone as GPS at this point is that it is not as weather resistant as the Garmin and you can't swap out iPhone's batteries when in the woods (but I think 3rd party charging devices might solve the latter.)

I'm curious about others' experience with the iPhone as their GPS and recommendations for other apps.