wow, some people are fucking retarded. the generic fisheye is shit, awful quality with terrible CA. Opeteka, Vivitar, Samyang, Bower, whatever its called, don't get it.
The Tokina 11-16 is arguably the best UWA on the market. That's why its resale value has gone up and also why it is being converted to PL mount. I've used it before, it is a fantastic lens. Little to no CA, great reproduction of colors and very little distortion. The 10-20's, 10-22, and other UWA's are nothing, NOTHING compared to the 11-16.