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?


Question: Should I get my guitar re-set up when changing string brands, but not gauges?
Answer: Same gauge strings = same set up. Same string mass, same tension, same speaking length. However... if you had the set up changed from original, which you liked, just changing brand will probably not do it for you.  Sounds like you need to revert to the original string gauge, which will require a re-set-up.