I wouldn't get one. I have a dead iPod Touch that was given to me and I got my key and rubbed it on the screen a couple times ... no scratches. Historically, non of my iOS devices scratched, and that was the only time I TRIED scratching it, and nothing happened.
Sure they are cheap but they ruin the screen quality (its not as enjoyable). If you get the cheap ones on Amazon, you'll see a spectrum of colors when you shine light across it so if you do get one, consider the $15 dollar ones ... you get what you pay for.