I am using the invisible shield screen protector and I like it a lot. My phone case from ATT came with a screen protector that seems to work pretty well too.
i have been using a screen protector since i got an iphone more than a year back. i really like it and it actually makes the screen more easier to use by not allowing your fingers to "stick" on the glossy screen. but get a matt finish protector rather than a glossy one.
the only diff might be a slight (very) decrease in the brightness after putting the protector. but thats any times better than a screen full of scratches and finger marks/swipes.
i recommend the INVISAsheild give it a google i have been using their covers for ever. got one on every expensive device that is worth saving the screen