I use
this one on my iPod Touch. It does feel a bit cheap, and the touch sensitivity is reduced ever so slightly(99% of the time, it's unnoticeable though). My friend, on the other hand, does not have a screen protector on this iPod and his screen is screen is scratched quite badly. For some unfathomable reason, he keeps his iPod in the same pocket as his keys. Needless to say, the keys win.
The way I see it is that the above-linked screen protector is so cheap that it's worth a shot, and it means you don't have to worry about being too carfull where you keep your iPod Touch. I'm definitely glad I have it. Just make sure that you take the time to affix it properly otherwise it will be noticeable and really cheapen the look of your iPod Touch.