I got an Agent18 Videoshield, which is a hard plastic (or polycarbonate) case. The screen does scratch easily, and there is no clickwheel protection. On the other hand, I've dropped it before without noticable results. Then again, if the case wasn't on, the iPod would probably have survived too.
What made me choose it over the Contour cases was that I'd heard the headphone jack openings on them wasn't big enough to get some plugs in. Dunno if that is the actual case or not.