I also have a 24/2.8 and I've used it on both a 20D and 40D with no problems. It's a pretty sharp lens. Perhaps a little soft in the corners when used wide open, but that's to be expected.

I guess you may have some sort of focus issue, though that's not common. You could try some of the tests that I suggest in this series of articles:

http://www.bobatkins.com/photography/technical/testing_lenses.html