Do external batteries for iPhones work well?
I have an iPhone 4S
For example, is it true that I can charge up to 6 times my iphone with this product?
Yes - and most people I know fit into strongly preferring a rechargeable solution when they use the device regularly to extend power on the go.
Those that use the device occasionally or on long trips seem to gravitate to an inexpensive sled for AA batteries and carry a mix of expensive lithium cells and typical non-rechargeable cells.