I tested all my lenses when I purchased a 5D2 last year and was initially horrified by the results from the 400mm f5.6.
Having worked on my shooting and processing, things have improved and though not as sharp as the 70-200 f4, I think it just shows what an exceptional lens that one is. I've stopped comparing the 400mm to my other lenses and just try to enjoy being outdoors and using it, since though this lens is “old” technology I don't think there is any alternative in its price and size range, unless Sigma pull a rabbit out of the hat with their new zooms.
I always use a a monopod or tripod with a 2kg bean bag sitting on top, and even then in a wind it's difficult to control, but there are are lot of wildlife shots I would have lost if I'd used another lens without its fast autofocus.
For landscape work I find the quality is good if there is no wind, otherwise the weight of my current tripod with the beanbag on top and backpack hanging from a hook is just not enough.
When I looked at alternatives to the 400mm, it worked out cheaper to switch to Sony, and the quality of the Canon 70-200mm lens is the main reason I haven't done so.