My battery was having some problems. It was draining pretty fast, it wouldn't last 2 full hours even if it was in 100% and also the battery was always hot, here where I love, in Dominican Republic, we don't have official apple stores so I took my chances and bought a battery kit!
Well, it was fine! I'm really surprised I managed to get it working! I think that the hardest part was the part of trying to lift up the screen, that was very difficult because I was doing it very careful, trying not to break anything! The battery didn't want to come out neither so I had to heat up the back a little bit with a hair blower until it came out easily! Now I can be with my phone ALL day without worrying that it will drain like it was doing with my old battery, very happy with this!
My Advice would be: Read the guide in this page very carefully, and also see the video so you won't miss anything. I was so close of taking out the screws and put them all together without knowing that they need to be exactly where they are before you take them out, otherwise you can damage something and the phone will have some problems. First of all I didn't know that the screws were different and they need to be exactly in the places they were. Oh God, good thing that I decided to read and see the video from this site BEFORE doing it.