I use these devices much the same as Brian Le. If I want to make a track of my route I'll use my GPS and bring some spare AA batteries for it, but I still bring my phone because getting a signal is pretty easy here.

If I only bring my phone I keep it turned off and only turn it on to send a text, check for messages and the weather forecast, or grab my lat/long.

I just got a new iPhone and the battery life in it is a huge improvement over my old Motorola Razr M, and it charges a lot faster (several times faster). I'm not recommending an iPhone, just noting the difference. I haven't used it on a backpacking trip yet.

I generally bring both devices because I like having them, but where you have some distinct topography it's pretty easy to know or find out where you are if you pay attention and keep your map handy. This is especially true here in the Ozarks where there are lots of peaks and creeks and hollows to reference and because of that I generally keep both of them off. This makes the dead weight, and all together it's not an insignificant amount of weight, but I choose to include them.

For me, these devices are a part of my "backup plan". But my primary plan is to pay attention to where I am and confirm that with my topo map and compass if I need it. Used this way, these devices can help you improve your mapping skills by confirming you are where you think you are, or instantly letting you know you're not, which should provide some clues to where you screwed up. After a bit you'll pretty much always know about where on your map and that should really always be the case.

Learning to read the topography is an important skill to acquire because thoughtlessly following trails is a pretty good way to get lost. GPSs have really helped me get better and have more confidence in knowing where I am and locating it on a topo map. And I think it's fun to test myself this way. I take a break, turn on my GPS and while it's getting it's location set I pinpoint where I think I am on my map. I've gotten pretty darn good at that now. You just have to pay close attention to the topography as you travel and a topo map tells you what to expect.

I also try to estimate my lat/long on the map and compare that with what my GPS provides. I don't pencil out the math, just a rough guesstimate. But, I'm maybe a bit odd that way. This kind of stuff keeps me amused.