I had one, and found it difficult to use, it is very long and requires a very sturdy tripod or fast shutter speeds. I had no problem autofocusing with my Canon 40d, even in low light.
The lens is not the sharpest there is, the Canon 400mm F:5.6 L makes it look a little fuzzy. Consider the Canon 100-400mm L is a alternative if you want zoom.
