Got the Nilkin (the matte one, not the glossy one), the Rock Quicksand and that official Sony hardcase (
http://www.gearzap.com/official-sony-xperia-s-hard-shell-case-black.html)
I'm very disappointed with the original one. It's almost impossible to take it off within 10 minutes without breaking it. Needless to say, I broke mine. Very fragile case while the Nilkin and Rock case offer about the same protection in a way easier and compact case.
I didn't like the material used in the official case either, it just doesn't have a nice feel to it. If I had to pick the odd one out and got hinted that the odd one out is a dealextreme case I'd pick the official hard case as it feels the cheapest overal.
I prefer the rock quicksand case simply because of it's texture, VERY prone to fingerprints and it just looks sexy and feels AMAZING in the hand. The Nilkin one isn't bad either, if I were to ever drop my S I'd hope i'll be using the Nilkin case when doing so. I swap between the 2 from time to time, I love change.
I would recommend the Rock and the Nilkin from ebay. I paid 18 euros for the official one, 10 for the rock and 10 for the nilkin. So I would not recommend the priciest one as it feels the cheapest in my opinion