I still use the snot out of my phone. I bring along one of those little battery recharger things if I'm going to be out a while. If you download the maps ahead of time on your phone they stay with you, whether or not you have cell service. Your phone has a built in GPS and doesn't require cell service to operate (at least if you have any of the last couple generation iPhones).
I use mine all the time and its great to have topo maps, aerial maps and whatever you manage to download before you lose service. It hasn't let me down yet.

I usually bring some paper maps as well if I can. The green trails maps are awesome.