Im using the belkin slip case. I quite like it.
I use it normally inside my pocket, like a pocket within a pocket sort of thing. It's quite comfortable to have my iPhone not shuffling around in my pocket when I'm walking about.
Also, when I need to actually look like "upper management" at work I just clip it on to my belt and it instantly adds that "tech savvy executive" look. LoL. I work in creative services at my company so I don't have to always wear a suit if I don't want to, but I'm always amazed at how "professional" a stupid cellphone on your belt strap can make you look, hah, another reason why I continue to wear mine in my pocket!