With budget often the deciding factor in choosing a big lens, you might take a look at the Sigma AF APO DG HSM 150-500 mm F5-6.3. I've been shooting one on a Nikon D300 lately and, apart from a rather noisy OS motor, I've been pleasantly surprised. The OS really does buy you at least a couple of F stops and has allowed me to shoot some stuff handheld that I could never had shot before without a tripod. With the crop factor on that camera, the lens is effectively a 225-750. I have no idea how it might perform on a Cannon, but it works very well on a Nikon.
