By the way, I was riding with a friend at the weekend, who is trying to use his iphone for logging rides, knowing his current average speed, distance, metres climbed and so on - (he has no plans for using it for navigation but he's training for an event so wants to monitor progress). The limited battery life completely undermines his efforts, to the extent that he is considering other power options (spare battery pack etc). I don't know what software he was using, but I think it has an option to sample position less frequently to use less power, which he had set. All the same, his phone died during our 80 mile jaunt but my ancient 60CSX was still on 2 bars from a couple of tatty old 2000mah rechargeable AAs at the end. At the moment I think that's a critical difference in useability. It not only failed to record the whole ride, but it left him without a phone afterwards, so he had to borrow mine. I suggested that a second hand etrex or similar off ebay would be his best option.
And, by the way, we were always in reach of a good mobile signal. If we'd been out in the wilds and his phone had been struggling to connect to the network things would have been worse for him.
The do-it-all single device is a great concept, but it's also a single point of failure. You want to keep your phone working for emergencies, whereas if your GPS dies you can revert to maps, roadsigns etc.
While I'm riding I put my phone onto battery save (2G only) mode and it lasts for days or else switch it off, while I have enough in the way of spare AA batteries to last my GPS for days.