My iphone 3gs kept shutting down every few hours while on calls or texting. It got worse and worse... to every 30 - 45 mins. Apple wouldnt do anything because it was out of warranty because I didnt call 2 months before. I hated the black screens I got so much that I was about to throw it out the window.
It went fine when I had all the pieces. I had to reorder the metal spudger because the plastic one will not cut it. The Suction cup was ordered also because its not available in the battery replacement package. Which is a travesty? Make sure you order all pieces because you want to do it right the 1st time.
Make sure you buy the metal spudger and suction up along with the original battery replacement package. Otherwise you will regret it!! Take your time when doing it and watch both youtube video and read ifixit guide to make sure you do it right. Otherwise you will regret it.