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. At comparable apertures the prime was slightly sharper, but below F5.6 it was not at all sharp and exhibited major cyan Chromatic Aberration. By comparison the 50mm F1.4 was far sharper at wide apertures, and only suffered from purple CA at F1.4.
This may be a bad copy of the lens, but I suspect these mid range primes no longer cut it on digital bodies (especially crop bodies). I'll be returning mine and going for a 135 F2 L.
To be fair I do have different requirements from this lens than most users, I wanted it for astrophotography and chromatic aberation is not pretty in a starfield. The testing I did was in day light however, with a tree against a white (UK!) sky. Most people with portrait or general photography needs wouldn't see the CA, but even so the fact that I couldn't get sharp results below F5.6 meant it didn't offer anything the 75-300 zoom didn't.
And yes I did all my testing on a solid tripod, using mirror lock up and a cable release.
Hope this helps.