I realize many say they aren't necessary and they may be right for their application. However, I gave my daughter an Ipod touch when she was 13 and put a screen protector on it right away. The difference between her screen and her friends without protection is like night and day. Granted not everyone bangs their stuff around but it's nice to be "covered" just in case.
I don't use one. The glass is sturdy enough that I can trust it won't shatter under normal circumstances (a screen protector wouldn't do much to help that anyway) and I have zero scratches on my screen. I used to be a big fan of screen protectors but I see no point in having them on the X.