I'm using an Incipio Frequency on mine. I also have a bumper coming on Tuesday. I would love to go bare, but I slide my phone into a custom car holder, and without a case my sides would be scratched to hell. Also, bare these phones tend to slide on smooth surfaces, so I want something with a little grip as well as a lip so I can set the phone on the screen and not scratch it.
I like the Incipio, though. It's sleek, quite thin, and is semi rigid. While thin, it really feels like the material would offer truly excellent protection in a drop.
I can't do bulky cases like the Otterbox...they drive me nuts.