I have one.
For me, little bits of something work their way under the case around the click wheel, scratching the surface of the iPod (though none worked their way up to the screen, so the most important part is in great shape.)
On the other hand, I noticed another guy at work with the same case and his iPod is pristine. I'm not the type to be careful of exactly what I put in my pocket with my iPod, so I guess I'm getting sand or dirt or something in my pocket with the iPod? That guy at work said he doesn't take any special care with what he throws in his pocket with his iPod so it might just be me.
When I do watch videos or look at photos (not too often) I po the iPod out of the case for the best viewing.