I have an invisible shield full body skin and yesterday I purchased a black rubber incase from the Apple store. Not sure yet if I'm going to use it though. What I really want is something with the design(covers bezel but not screen)of the incase and agent 18 but in a much slimmer form factor. I checked out some of the employee's iPhones with the agent 18 case and it was a little too bulky for my liking. I knew the incase had a similar look and I bought it in hope of it being thinner. I could not test it out because it was sealed and none of the employees had one on their iPhone. It fits and protects good but I would really like something slimmer.
I have the Case-Mate flip case with holster from here, http://store.everythingicafe.com/content/accessories/4-22--2742.htm. I love it because it gives me a nice very slim case (it adds basically nothing to the size of the phone) in case I want to carry it in my pocket, but also gives me the option of a nice belt holster to hold it with. I'm a huge fan of it.