I bought an used iPhone 3GS as gift for my wife. And got a device with ugly fissures on the white rear case and a geriatric battery. So I decided to change the back and - on the way - renew the battery.
Following the guides step by step, everything worked well. Differing from the good and clean photos, I found a lot of dust & dirt and needed al lot of time to remove it.
Not expensive, but very practical: the spudger which I ordered separate from the one added to the battery pack. With its sharp end it is quite easy to lift tiny parts or cables.