If I feel the need to carry a GPS, my personal preference is to carry a dedicated GPS (Garmin in my case). I will sometimes also carry my iphone, but I look at it as strictly a back up for the Garmin.

My reasons for not relying on the iphone as my primary GPS are listed below. I do realize there are ways to mitigate some of these issues, and I've noted some of those.

1. Most smartphones are rather fragile and are not waterproof. In back country situations it is all to easy for a smartphone to be knocked out of action, probably when you need it most. However, one could put a smartphone into a protective case, and there are some available that are more rugged than the iphone.

2. GPS tends to use a lot of battery. I can easily replace the batteries in my Garmin in the field. Recharging my iphone requires getting back to a plug in. Again, there are options to recharge smart phones in the field, but this means carrying more stuff with no other purpose.

To summarize, in the rugged areas I frequent, I get by 98% of the time by using terrain association with a map. I always carry a compass, but seldom need it. I often carry a GPS, mostly for fun, but now and then it can be priceless (in a whiteout for example). I often carry my iphone (in a ziploc bag, deep in my pack), but consider it only a back up.

Those are my personal choices. As always, your mileage may vary.