I have two iPhone 3GS with a broken screen and a battery that drains almost instantly after a full charge. Decided to give it a try fixing after watching some videos on the site.
It didn't go as smoothly as compared to the video - but was able to find a way to put things back in order.
The replacement battery that I've got will not fix easily on the sides as compared to the original battery. It will not align properly and is slanted so I spent sometime figuring out how to make it align to the sides. But to no avail. I left it at a slanted position but was glad that it worked.
Just hope that the parts are the same as the original. Easy to fix back and will not give you trouble.
iPhone 3G and 3GS Volume Button
iPhone 3GS Display
iPhone 3GS Battery
Pro Tech Toolkit