I own this exact lens, the Canon 15mm f/2.8. I love it, it's a great lens. I had the opportunity to try it out on a 5D MKII and the distortion is sooooo much more noticeable on a full frame!! Not used to so much distortion since my XTi has a 1.6x cropped frame. Anyways, if I were you, I seriously wouldn't waste my time and money on a screw on fisheye. The quality is shitty and you have some noticeable vignetting and blur in the corners.
The 15mm is a lens designed with intentional distortion, has excellent corner sharpness, and very very nice image quality! I highly recommend it!