My iPhone 4's logic board died and I did not want a new wireless contract at this time. I found my old iPhone 3 and began using it. The battery would only last 1-2 hours. Another person on my family plan has an iPhone 3GS and her battery would last about the same amount of time. So, I replaced both batteries.
The repair went extremely well. I was nervous that I would render the phone useless (by breaking something else) but it is in great shape (as of this exact moment)!
This is a cost effective way of extending the use of an old cell phone. I want to wait for the iPhone 5 (or whatever they are going to call it). Now, I have some life in my old phone so hopefully it will last until Apple releases the next new thing! Thanks!