I got my 4S about a year ago when it had just came out, and I was one of the users plagued by the firmware-related battery issue. I'm pretty sure the constant cycling killed my battery prematurely, so I decided to replace it so I could actually get more than a day of light use from it.
The repair went pretty well. I was a little nervous though, since everything inside the 4S looks like it would break if you just looked at it a little too hard.
It seemed like the factory went a little glue-happy, so the original battery was stuck in there pretty good. Don't be afraid to use a little force to get the battery out; it'll make some horrible crackling sounds, but it's nothing to worry about. Just double-check the guide to make sure all of the connectors are detached.