Well my cousin has been using his phone (GS2) for almost 6 months without ANY protection at all (No cases or covers and no screen protector), he doesn't cares about the cellphone at all and there's only like 2 or 3 scratches (Just light scratches) in the screen while the device itself is all scratched
So i would say it's pretty good.
On my side since day 1 i always had a case (Similar to Otterbox but by Trident) and a screen protector, my device still looks brand new . . . the only problem is the Home button, even being careful it gets microscopic scratches.
BTW, a few days ago i purchased a SGP Ultra Clear for my GS2 and it's AWESOME. Recommended.