Anybody with a smart phone knows only too well that the battery will not last all day. I have been looking for a backup battery for a while now. I chose the Powertraveller Powermonkey Discovery Portable Charger, because it has a metal casing and looks robust. This backup battery will charge your iPhone twice and which is more than enough for a days use outside the home or office. It comes very handy when you're only holiday. Sure the larger solar powered models may have more capacity, but there isn't really that much sun in the UK and it would mean carrying a large unit and would need to be charged via the mains anyway. The size of this backup battery is no bigger than your 90's candy bar phone and great if you're a believer in travelling light. It will charge up your phone at the same speed as the mains, but you would need to use the charging/USB cable that comes with your phone.
What I don't like about it is that it is a nuisance to get charging. It doesn't always work the first time. It seems to work fine in the UK, but when you're abroad and you're fiddling with it for some time, it is a bit annoying. Also, a full charge takes several hours, so you plug it in before you go to sleep and it will be ready by the time you have breakfast. I would take a spare travel adaptor with you as you're probably going to need to charger up other devices or your camera batteries. Another thing I don't like about it is the lack of a lead for iPhones. You get a whole host of adaptors, which look a bit flimsy, but nothing for iPhones or iPods etc.