I also have a slim leather Sena sleeve which is more appropriate in formal circumstances (black) than my Pixelskin (blue).
iSkin Solo for me. It's extremely thin and lightweight and has amazing quality. Has never failed me. Don't believe all the hype about Otterbox! Bulky, heavy, and cracked on me from much less falls than my iSkin has taken.
I skin is a great company. I had the Solo for my iPhone 3G but use the Revo 4 for my iPhone 4S. Check them out.
I use an Otterbox Commuter and like it.
I finally ordered my iphone 4S, but now I'm not sure what case I need to buy, anyone know some good cases?