The Innocases are pretty great; thin, and they add much-needed "grippy" texture to the phone. The actual N1 surface is pretty slick if you have any sweat on your hands (in my experience).
I didn't want the "overkill" of an Otterbox, so I have the Innocase that lives on my phone, then the appropriately-sized belt clip from Seidio. It covers the face for full protection when I need it, which isn't often. I only use it when I'm doing something high-energy, and at that point, being able to securely attach it to my belt rather than keeping the phone in my pocket is a bonus.