I'm using the Spigen Slim. I like it so far, it adds very little bulk. I don't expect it to provide tons of protection, I am more looking for it to protect against scratches and the like. If I put it down on a rough surface, I won't have to worry about the phone.
One complaint is that it leaves a vertical ridge around the screen. That lip that comes up around the screen isn't rounded off, it just kind of sticks up.
A benefit that I've realized that most cases likely provide is making it easier to find the buttons on the back. If I have it in my pocket while listening to music, with the buttons facing out, having a case on it gives me an tactile extra clue where the volume buttons are. Even through my pocket, I can find the depression for all the buttons, to help find the volume button I need.