I had purchased this for my 5D because the 135mm/f2L with 1.4x teleconverter (189mm) was usually not long enough for field sports such as baseball or soccer. This is sharper than my old 70-200/2.8L/IS zoom, and it's very lightweight (2.8 lbs). Great for distant candids. A real gem for this price range ($1050). When hand-holding, try to keep the shutter speed at least 1/800. With f/5.6, you may need to increase ISO. It would be great if the lens were f/4 with IS, but it wouldn't be in this price range any more. f/5.6 does not provide enough bokeh, but it's very sharp. The minimum focusing distance of 11.5 feet can be annoying for close-ups sometimes. The lens focuses very fast, by the way.

I recently sold this and purchased the 300mm f/4L IS because I need something more often in the 200-300mm range, I want something that can get very tight head shots without an extension tube, and the extra stop and IS come in handy in low-light situations. My conclusion is that it makes sense to get the 400mm only if you know you'll almost always need at least 400mm. This typically means you are shooting wildlife or field sports in good light. In the end, I felt that I needed 300mm more often than 400mm even with my full-frame 5D, so I'm willing to compromise with the 1.4x TC when needed (420mm). I'm going to miss the sharpness and fast focusing of the 400/5.6L.

I recommend the The-Digital-Picture.com for comprehensive reviews.