I agree with Rod - you must have devices with removable batteries for anything more than a day walk. I've got a dedicated GPS, but this year I'll be using a Samsung Galaxy III - weight with two spare batteries is 200gm, which is the same as my Garmin ETrex with spare batteries.

The majority of smartphones - like iPhones 4 and 5, Sony Experias S, T and Z, are solid body phones, which means the battery can't be swapped out or changed. In my opinion this makes them completely unsuitable for any extended trip.

My mapping software provider charged me £6.99 for the Android app, and because their storage is cloud-based I was very easily able to sync and download all my current mapping (UK North 1:50K, the majority of the Challenge area at 1:25K and the whole of the French Pyrenees at 1:25K) to both the smartphone and a Google Nexus 7 tablet. This works a treat - you've got an excellent GPS, all data is held on the phone's SD card and no need for a data or Wi-Fi connection.

The HTC Desire S is another good smartphone with replaceable batteries.

And this morning I found this amazing Android app that turns your smart-phone into a full-on GPS unit. For free.

Click here for details