My 3-year-old iPhone 3GS' battery wouldn't even last for a full day with regular use, so I decided it was time to replace it while I wait for the iPhone 5 to come out.
The guide is very detailed and easy to follow, did everything step by step, being very careful, and it all went well.
Releasing the battery was harder than I thought. The adhesive was pretty strong, so I needed some good hot air from the hair drier to soften it, but even with that it still required a good amount of pressure. I didn't just use the spudger from the back of the battery as shown in the picture, I also ran it along the sides and applied some pressure there to release the adhesive.