I use this with my iPhone 4. I am planning a cycle trip form John O'Groats to Lands End and I don't want to take maps, but I also don't want to get lost. The iPhone has sat nav and all manner of useful tools on it - but the battery doesn't last when you have the satnav on all the time. I bought this to see whether it would help - it does!
I can put my iPhone on my bike with a full charge, use the satnav for over three hours with this plugged into it and the iphone is still on full charge. You can tell when the batteries run out, because the iPhone tells you the device is not compatible as soon as there is too little juice in it. Simple, stop, replace batteries and off you go.
I think it is better to use it this way than to try to use it to charge a device from flat - It will work like this, but I suspect it will not be as efficient.
As per the previous review, the battery cover is a bit of a fiddle to get off, but not worth knocking a star off for it. For £4 it really is a brilliant little device.