okay, you want to love this lens. it is a prime 400 and it is light and easy to carry--especially on a plane if need be. in certain conditions--such as late afternoon or early morning with the sun at yr back, it hangs with the best canon offers (optically). with its lite weight, you can take it to places you wouldn't want to lug a 400 2.8 or 600. So if a transportable 400 is what you need, then carefully consider it.
so why a 7 ratings? for the money it cost, you can buy a 500 at the same speed and get a 'killer' lens. wonderful contrast, super sharp and not too heavy. yes it is longer, but still easy to transport. I have shot several versions of both the 400 DO and the 500 4.0 IS and the 500 wins on ALL accounts. On the other side there is the 300 2.8 IS--another 'killer' Canon lens that is equally lite in weight and world class performance. Oh it is MUCH cheaper than the DO. so get a 1.4X for the times you need that extra length. Or go for a 1DS or 1DS Mkll and crop to the length. Both ways with the 300 2.8 you have a much more verstile and better performing optic--and us sports guys love 'em because we can shoot night games with them.
the other option is of course the 'REAL' 400 in the Canon line-up, the 2.8 IS. This lens will beat the DO in every aspect except weight. When you see the Getty Pros, the SI staffers, etc, etc on the sidelines of the major sports events of the world, you see this optic 90% of the time--I RARELY see a DO, and most cases it is one that was checked out for the day from CPS for trial. the other 10% tend to be 600's, 500's and 300's.