Optically this lens is a champ. I am no optics engineer, perhaps designing a fisheye isn't that hard after all... because this one simply in another league with similar rectangular lenses I've tried: not only it's f2.8 (being relatively compact), but it's tack-sharp @2.8! Even flare isn't much of a problem. Colors, contrast, sharpness - everything is great.
However at such wide angle 10D's autofocus gets confused. Actually this lens can't AF on anything beyond 2-3 meters. But with such huge DOF I usually just set it somewere between 1m and infinity, and everything is in focus, even at f2.8.
Also I noticed that my 10D tends to overexposure about one stop with this lens. Not sure if it's a wide angle issue. I haven't used anything wider than 17mm rectangular, so I can't be sure.