The portable phone (or USB) charger seems to be a fairly new innovation, the premise being that it's something that you can use to charge up your phone, Kindle or MP3 player when the battery's running low, and you're not near a power socket. I've had this for a few weeks, and it's already proved to be invaluable, notably on a long train journey where my carriage didn't have plug sockets as normal. The charger roughly charged my iPhone to about 60% in a very short time, and thus meant that I could keep myself entertained during my much-delayed jouney.
So, the plus side - can extend battery life for your phone or USB device simply and quickly. The down side - well, firstly it's rather heavy, but I suppose that's to be expected for it to have a battery that will retain and dispense a fair amount of charge. The other main down side is simply one's memory - it's no good if you a) don't have it with you, or b) have forgotten to charge the charger. But, assuming you're organised enough to remember to do this, then it's a great buy.