Once you get to the edge of cell service and under tree cover, your iPhone will be nothing more than a hot brick as it burns through the battery trying to locate 3G service.
I started out with my iPhone on a couple of trips and it worked very well, but I always had 3G service. I tried to find some caches in the conservation land in town in an area that has not so great cell service. I walked 100 ft into the woods and lost the link to the geocache.com app. The GPS also lost lock and I didn't get either back until I came back out to the road. I had a similar issue where the electronic compass lost it's mind in the woods. You could rotate 360 degrees and the compass continued to point in the same direction.
I like to take the kids to new trails/towns and have no idea what cell service will be, so that's a deal breaker for me.
