I never use them. I like the screen to be 100% natural and screen protectors take away from that. I always leave my phones naked. I do however always have phone insurance just to be safe.
If I'm not mistaken these phones have gorilla glass on the screens which is very resistant to scratches which makes (IMHO) screen protectors not necessary.
I had faith in the scratch resistance of the screen but I got a scratch after 4 days of rocking the phone naked and being extra cautious to have it with nothing else in my pocket or dropping it. Its got a shallow scratch that you cant see unless its displaying white but still something to think about. Ordered a screen protector and kickstand case for it.
I just traded my 2yo Epic for an S3, and there wasn't a scratch on the Gorilla Glass screen. The S3 has the Gorilla Glass II, so I'm sure it's even better. No shield for me.