I think that the iSkin Duo is a great investment. I personally just ordered my 3rd one yesturday. They just keep coming out with more and more colors, I just can't help myself. I started out with the light blue/clear duo, and currently I'm sporting the blue/green duo. I just ordered the black/red duo yesturday as well as the matching headphones (my apple headphone's right side blew). I can't wait to get it.
Anyways, the Duo is the best protection that is available for the duo, imo. It's double covered and has a screen protector. Plus the connector covers are a nice touch. I admit, it takes maybe an hour to get the feel for the click wheel cover, but it works pretty good. If it bothers you that much, you could either cut it out, or just put on the outer layer for your cover, but then again you'll miss out on the screen cover if you do that. It also has a pretty nice belt clip, although the belt clip is a little bulky for the nano.
I, personally, think that it is a great investment for your nano.