I had an InvisibleShield screen protector on my iPhone 2G, and I didn't put it on very straight. I was afraid about the amount of fluid it recommended using to position the IS, so I don't think I used enough and it stuck before I was ready. I tried to pull it up, and it stretched a little so, it slightly hung over the earpiece speaker and onto the chrome bezel. That has bothered me since. But it worked very well as a screen protector and it is still on the phone over 1.5 years later, without discoloration or any sign of damage. I realized the application of the InvisibleShield is key. I used the Marware CEO Sleeve for my case, with the InvisibleShield, and I was very happy with it.
I now have the iPhone 3GS, and I wanted to do something different. I got the Incipio Feather to protect the back/body of the iPhone from scratches, and for more grip. It came with two screen protectors, so I applied one. I realized that I really missed the oleophobic coating of the 3GS, and the screen protector had a "rainbow" haze to it when looking at it from certain angles. I ended up taking off the screen protector. There is a huge difference between the 3GS screen coating vs the InvisibleShield or Incipio screen protector as far as fingerprints and smudges. And my iPhone 3GS fits in my Marware Sleeve with the Feather case on! (In case I don't want it in my pocket) That's a bonus!
If I decide to protect my iPhone 3GS screen, I'd strongly consider getting the InvisibleShield again, especially since they've gotten better since I got mine, from what I hear. But I wasn't so impressed with it that I wouldn't consider other screen protectors. Hopefully it won't get scratched before I decide!