Had a chance to shoot with the 200-400mm during the recent Canon Lion City Cup. Some quick impressions:
The 200-400mm f/4L IS next to it's tiny 70-200mm f/4L IS cousin.
Overall I think it's safe to say this is Canon's sharpest zoom lens (and most expensive while at it). Pretty hand-holdable, if you're the kind that thinks a 300mm f/2.8 is hand-holdable. For football, the zoom range is very versatile. You can get action from way across half-field AND just within the penalty box. With the flick of the built-in TC, across the field goal mouth action is *just* possible.
Handling wise, the zoom ring has quite a heft to it, you'll need your entire hand on the zoom ring to move it with any speed. Unlike the more common 2-fingers under the lens to zoom technique most people use. The TC switch needs some getting used to flick without leaving your eye off the camera.
Singapore U15 vs Corinthians
Waiting to hear what the others have to say on the 200-400mm.