My buddy put one on his TZ and it played terrible. Really changed the game. Now it could have been that his game needed further tweaking. He ended up removing it and installing a new playfield.
Another friend installed a protector on a NIB Dead Pool and Iron Maiden. In both cases, it hasn't changed the play at all. I really like the protectors for new Sterns.
For those that say a protector in not needed in home, I disagree. I've seen HUO Sterns, most notably a couple of Trons, with wear on inserts. Without a doubt, Stern playfields are not up to the quality that Williams were especially newer Sterns.