I just recently got a canon 400mm 5.6L, so i havent had more than 2 weeks to shoot with it but so far...I LOVE IT!!!!!!! this photo was shot yesterday 1/27....
Look at these: http://www.the-digital-picture.com/Reviews/Canon-EF-100-400mm-f-4.5-5.6-L-IS-USM-Lens-Review.aspx http://www.fredmiranda.com/reviews/showproduct.php?product=19&sort=7&cat=27&page=1 Remember that this lens is not compatible with the Canon extenders for auto focus with the 20D.