Summary: I have the Cosina version of this lens which is very sharp and have good contrast wide open at 400mm (even with my Nikon TC-16A tele-convertor) on my cropped sensor Nikon D70 body. This might not be the case if you use full frame camera. Auto focus speed is reasonably fast at 400mm F6.7 and purple fringing for high contrast areas is quite tolerable.
Summary: I love this lens. That said, my first day out I was disappointed with the lens creep and it seems to have some vignetting though not pronounced in the final pictures. Tightening the ring made for less fluid zoom, still figuring this one out. However, the images I took were clear and close, nice quality and beautiful colors. This lense replaces an old 70-300 Tamron zoom, the glass is much more sharp, and the focus seems more precise.
Summary: I have spent a lot of time in India's jungles trying to capture its wildlife as digital images. The results with my older lenses was always just mediocre. The Canon EF 100-400mm lens has changed all that. I am now getting beautiful images with lots of detail.
Summary: As an L rated Canon Lens, it performs as would be expected (SUPERB). This lens is HEAVY and it's because of the quality put into this product. The real intent of this lens is for OUTDOOR USE, and as long as you remember that you'll always get great results from this lens. I would recommend a tripod/monopod when using this heavy unit. I highly recommend this for anyone wanting high quality pictures taken at a distance.