I don't see many comments on lenses over 300mm. Is it not that common for people to buy 400mm?
Still expensive by comparison to most common lenses, but a perhaps more practical proposition would be the EF 300mm f/4.0L IS USM at $1,119.95. It would also be a damned sight less of a strain in carrying it round too when compared with some of Mike's list. This one's on my list of potential future buys.