(especially metals).
Or sand. A single grain in your pocket or purse can do a ton of damage to any screen (gorilla glass, or not).
I bought a Zagg a few weeks back and have removed it several times trying to get it right, but I am amazed at how good the screen looks underneath compared to the first EVO LTE I had replaced with no screen protector everytime I take it off. They are expensive but I am now a believer.