Played a Bally Star Trek all weekend with a protector on it.
There were some issues. The game played strangely off the slingshots and sometimes had very odd English.
I think it's a good product for a game with very messed up inserts or other issues like that, but I don't see why you would install one on a playfield that's in nice shape. To "protect" it?
People are way too preoccupied with that. Games in home use simply don't wear significantly.