I had a Power Support Crystal film on my 3GS from day 1. Never had to replace it and the film still looked great with minimal markings after one year. Easily installed it myself and had no bubbles or flaws. Got my set on order for the iPhone 4 and can't wait! For now, I have the 3GS film temporarily on the front of the new phone. It's not a good fit, but it's better than nothing!
On a side note, I'm considering just using the films with a bumper. Always had a hard case to protect the 3GS, but, with the camera flash issues, I am a bit weary of having anything on the back. Never thought I'd consider using only a film on the back.