I've had a few cases so far, here are my thoughts:
Belkin Folio: I was completely sold on this case for a few days. Really nice build quality, protects the phone well. But the bulk eventually got to me, and I returned it. If you don't mind the size, I think it's the best case available.
Incipio Feather: Very nice looking case. It doesn't protect the camera at all, which is a big negative for me.
Nokia Official: Not nearly as flimsy in person as it is on video. I was quite concerned about the cover flopping around, but it really doesn't that much. I'd still like it if it were magnetized. For some reason the back is covered with smudges that are very difficult to remove.
Amzer Pudding Case: I love this case. I'm not crazy about the glossy black look on the sides; I think it makes it look cheap. I would also prefer that the buttons not be covered, as I feel the covers make the buttons mushy after time. However, it protects the phone and camera from normal wear and tear, and doesn't add much bulk, which is what I want in a case. The price is great as well.