20 - 30 minuten
My 3GS was crashing all the time (a common problem after updating to iOS 4, see https://discussions.apple.com/thread/254...). Sent it in for "repair", but after five weeks in the shop, was returned with the same symptoms. After discovering the linked thread, I ordered the battery, replaced it, and no more crashing!
Fairly well. I had sent in my phone twice previously for repair, and I think they had damaged the glass, as gently pulling with the suction cup resulted in separating the glass front from the display module instead of the display module coming with it! After some fooling around, I had to resort to a small screwdriver to pry up the display module. Afterwards, the glass doesn't sit quite as flat, but that's something I could fix by replacing the adhesive strips and remounting the glass. Aside from that, it was all good!
Just keep in mind the possible need for a small screwdriver to pry up the display module in case the (superior and safer) suction cup technique doesn't work for you! The included plastic tool just wasn't enough to pry up the module in my case, although, as I mentioned, it's possible this may have been the result of some abuse in the shop.