This is a solution I really need. I often hike in the country and am away from power sockets. I also have a thirsty Nokia N95 which will flatten the battery in 2 or 3 hours playing MP3s, so this seems like a good solution. I bought one at an airport and, like another reviewer, another at Glastonbury Festival 2008 so my opinion is not based on one flawed model.
Firstly for some odd reason the mini Nokia plug seems to fit in such a way that it disconnects itself easily so that you have to jiggle it until the charge light comes on. In fairness that seems to be the fault of the Nokia plug type not the PowerChimp but it doesn't help because if you stuff the phone and charger in your bag thinking it is charging it isn't. This doesn't happen with the mains charger for some reason.
More problematic is that it charges amazingly slowly. Using the mains charger you can fully recharge a flat battery in an hour and a half or thereabouts. Using either rechargeable (2500Mah) batteries or Duracells the charger* says* it is charging, but even after many hours you seem to get only one or two bars on the phone's battery. Why this is I don't know: cheap electronics that don't let the phone draw power fast enough while *still* flattening the charging battery? Also when the phone battery went completely flat the PowerChimp wasn't able to revive it and I had to spend a day without a phone. Bad.
This device let me down totally - it blows. Trouble is there don't seem to be any similar devices that do a better job (Though at Glastonbury people were reselling what looked like a rebranded version of the same thing - a Thumbs Up device also available on Amazon - that is in a slightly larger blue metal case but that also sucked: Thumbs Down in fact).
It's cheap & so it is worth a try, you might get lucky, but don't rely on it.