I purchased a used iPhone and it needed a new battery.
Everything went smoothly besides the adhesive strips that came for the battery: they would not come off of the protective plastic that they came with. Battery is still secure without the adhesives though.
Buy the tool kit it will make things a lot easier. Anyone who has done any sort of repairs before knows that having the right tools can be the difference of hours. Follow the repair guides, be patient. Go easy of the caffeine beforehand.