I have this lens and can tell you it is a great lens for the price. It has done me proud on many occasions with my bird shots in particular. I have had no problems with chromatic aberrations :shrug: As with any lens, getting to know it and it's sweet spot pays off. Granted the APO version would be better, but I wouldn't write off this lens at all.
This was one of my best shots using the very same lens, you can click it to view a much larger shot
:
IMHO of course
Edited to add: It work fine using auto focus on my Canon 350D and also then my 40D, so I don't think for a second you would have a problem with your 400D Dion