Conclusion: Well,....if it weren't for the CA,...this lens would be pretty nice to work with.. It was noticeably not as sharp as my 85mm f1.8 either.. The CA was noticeable up to f4.0..and wide open, it was so noticeable that it made shooting wide open unacceptable. I've shot with many different Canon lenses from fisheye to super teles, and this was the worst as far as CA goes.. I ended up selling it after it sat for 6 months without using it..
Pros: Nice focal length for portraits / head shots.. Fast, smooth AF.. Build quality is good..
Cons: Useless wide open..because of the CA.. Not as sharp as 85 f1.8
Conclusion: I purchased this lens and a EF 50mm F1.4 based on the reviews here. I was looking for a good quality, fast, medium telephoto lens for my 20D body. The build quality of the 100mm if much better than the 50mm F1.4, but I cannot recommend it with a crop digital body. Testing this lens against a EF 75-300 F4-5.6 zoom, should have been no contest, but at 100mm the zoom gave the prime a run for its money.
Pros: Small size and light weight, solid build, excellent from F5.6 on
Cons: CA and lack of sharpness below F5.6 on EOS 20D