Having just upgraded to the iPhone 4S, I wanted a decent screen protector, but didn't want to pay a fortune. It seems you can spend as much as you want on these things, but every single one has mixed reviews and you can't be sure of what you're getting.
I've bought screen protectors from this seller for my previous 3g iPhone and found the product excellent, so thought I would try them again. I wasn't disappointed. The service is great, the pack arrived within a day or so and the screen protectors are perfectly adequate, they cover the phone's screen perfectly and hold nice and tight, without gaps or slippage. Once they are applied on to a clean screen, nice and straight, they'll stay put for ages.
As long as you are patient with the application of the plastic screen savers, you can't really go wrong. There are ten in a pack and for the price, it doesn't really matter if you do make a mistake once or twice. I wouldn't hesitate in recommending these to other users.