Rating: 5 / 5
I have been using the iPhone-4 (ATT model) as a gps/mapping device for six months now, including using it as my navigation device for a six week hike.
* Pre-downloaded all maps and all tracks and waypoints via WIFI before trip, so that no data connection is used while hiking.
* Display size and resolution is fantastic.
* There are some fantastic map/gps apps available (and also hundreds of crummy or inappropriate apps).
* The iPhone does much more than just map/gps: bird guides, multi-lingual dictionary, reference material, tide charts, etc.
The iPhone has been working very well for us and we are thrilled to have it. However, in order to use it successfully we had to figure out, and then tend to, the details of battery conservation, which is not obvious. For a careless or naive user battery management will make it impossible to use for backpacking. For me, now that I've figured out how to tune the battery life issues, it is a five star tool. For a naive or careless user, the 5 star rating will not hold up because the battery is likely to drain in one day of use. During our six week hike, we used between 7% and 20% of the battery per day (in addition to gps/map we were using it for bird guide, Turkish-English Dictionary, and reading reference material). We recharged at shops along the way. We wrote up instructions on how to use the iPhone for backpacking which includes detailed instructions for managing battery life.
Also, we waded through over 60 apps to find the ones best suited to backpacking - that was an enormous investment of time. In reality, the research we did to figure out how to use it for backpacking was by far and away the biggest downside to the tool. Unlike a Garmin, it does not just work "out of the box". We're maintaining a (hopefully current) annotated list of gps/mapping apps
I bought an iPhone specifically to use as a gps device. Fairly expensive initial investment, however the apps I'm using are inexpensive and the map content is free, so the ongoing costs are nominal. For somebody who already owns an iPhone, the cost of buying the apps (and their map content) is trivial.
There are a dozen good and useful mapping/gps apps that allow you to pre-download topo maps for offline use. I'll review a few of the iPhone apps as separate entries. In addition to gps/mapping apps, there are several apps that I like a lot when preparing for or during the hike:
Read It Later - to save wiki pages about the sites along the route in order to read them offline.
GPSBabel - desktop tool for manipulating gps data files.
DropBox - for sharing files between my iPhone, desktop computer, husband's work computer, and with friends.
Good Reader - For route information, camera manual, etc.
Shralp Tide - Tide charts for many countries.