I've got a black Core case for both my nano and 5G. For my nano its still my "go-to" case... its small, has brilliant protection (aluminium with neoprene inner, and a screen protector) and looks great with my black nano. The 5G Core is still a case I use, but since Contour finally released the Showcase Video, thats my number one case. I have a Moshi Pouch for "naked" iPod usage too. I may get one of Vakaadoo iVak 5G cases as I think they may be quite good.
I've also got for the nano: Macally Icesuit (black and orange), Power Support Crystal jacket, Moshi Pouch, C6 MFG Carbon Fibre Shell and I'm seriously considering ordering the iWood in mahogany. As you can tell I'm a bit of an accessory junkie! I've had more cases but sold them on as they didn't make the grade IMHO (iSkin Duo for nano, iSkin eVo3 for 5G, and Contour iSee for nano).
I have the Power Support Crystal Film on both of my iPods, and despite them working brilliantly (although I've had to replace the back film of my nano once) I think an additional case - or at least a decent soft case like the Moshi Pouch or Sock is the way to go...