Currently own Canon EF 70-200mm f/2.8 L USM, amazingly sharp lens, fast autofocus. I required more reach for birding, 200mm just wasn't enough, and from what I had seen, a 2x converter was as good as going off and buying a cheapo mirror lens, or even upsizing in photoshop!
This 400mm lens is lighter and thinner than my 70-200, which makes it so much easier to hand hold. Due to the double focal length compared to a 70-200 at full zoom, any movement is magnified. I can still shoot handheld at 1/250 with 50% success! No image stabilizer!
The autofocus outpaces the 70-200, although the f/5.6 minimum apperture means that a little hunting is sometimes a problem in shadows. Fulltime manual focus sorts this out, as one can tweak, then snap the auto for accuracy, if required.
Optical sharpness is fabulous. Compared to a "re-adjusted" Tamron 200-500mm, which was previoulsy unusable from the factory, the Canon wins contrast and sharpness. At f/8-11 the Tamron is at its sharpest at 400mm, the Canon compares wide open. Stopping down to f/7.1 on the Canon, ultimate sharpness and contrast is achieved, and continues through f/18 or so, just depth of field increases.
The detachable tripod mount is a useful tool. The 70-200 f/2.8 doesn't have this, where the f/4 does I believe. It just allows much easier connection to a monopod or such, and also allows it to be reversed for comfort. I reverse the tripod mount, so the foot faces the camera, and turn this to a 90* angle. This rests in my hand very comfortably and aids stability.
I find this lens an absolute joy to use handheld, on a fixed monopod, on a monopod with tilt head and with a monopod over the shoulder technique. I can't be damned with a tripod, too big and too much weight to take on a hike. I don't find f/5.6 a problem with images, as frequently, 400mm does not give much deth of field at anything below 20ft. I tend to be at f/7.1 and above, and with a decent Canon d-SLR, ISO 800 is no problem.
Hope that helps