I'm going to have to disagree with everyone.
Yes, the invisible shield screen protector will affect the performance of your S2. However, I would still recommend it. It will take a little time to get used to but the layer of material between your finger and the screen makes definitely hinders the accuracy of the screen. Not so much that is will bug you, but from time to time you will notice it. Another thing, the material of the screen protector will make sliding your fingers around the screen much more difficult especially when typing. Like I said before, neither of these issues are deal breakers, but they will probably bug you for a while and will require some getting used to.
Again I recommend them, but the have their downside as well.
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