I got a large pack of like 3 or 4 zagg invisible shields which you cut yourself to fit your gadgets and was not impressed at all. It's not really a "hard" protector, the protector is actually made of a soft almost rubbery material. It's a pretty impressive material and does protect from scratches pretty well, but the problem I had was that it's not invisible at all and looks pretty dumb. It also started peeling off around the edges after a a few months, less than even half year. The protector I placed on my psp also got this large mark on it for some reason, which is strange because I always keep it in a hardshell and baby it. Ended up giving my last protector to a friend for free.
Byt the way, on anythingbutipod, they do some tests where they scrape a bunch of glass touchscreens with razors and knives, and most of the touch screen players don't get any scratches or even marks. I personally haven't gotten any scratches on my ipod touch 2g yet without any protection too.