Summary: I have owned this lens for about three months now, and all I can say that NO other lens will even comes close to beating the picture quality of this lens. I have tried the new 600mm f4 ii of canon. It is a fantastic lens, but in order to get the same focal length you need to use 1.4x TC (slightly larger at 840mm). The picture quality is not comparable to 800mm. I have seen so many review on internet that the new 600mm would make the 800mm obsolete, but I beg to differ.
Summary: This is absolutely the best super telephoto lens that I have ever used. I own 300mm, 400mm version II from Canon. I also tried the new 600mm USM II. It was an excellent lens, but color rendition and quality of images from this lens even beats the new 600mm. If you need the focal length, then this lens has NO rivals.
Summary: I absolutely love this lens, I purchased a reverse mount adapter for this last week and have been using it for some macro photography. I've also just recently discovered a new microscopic particle way beyond the atom and have received nomination for the Nobel Prize. Well done as always Canon.
Summary: Stupid me! I didn't have a tripod worthy of this new lens so that was my next purchase. It is too heavy for hand held and just like any telephoto lens you will have to experiment to find the "sweet spot" settings that work best with your camera. If this is your first super telephoto lens just know that practice makes perfect in this case. I'm using it to shoot flying ocean birds from a deck.
Summary: I will admit, the last few months I have really been in a rut. Due to some personal issues I, a pretty average guy, ended up learning quite a few new terms. Like 'harassment', 'court order', 'stalking', and 'restraining order'. I am no legal scholar, I barely made it through the last 3 years of high school. But I can tell you this. I love my Lorraine, even if she is a little stand offish at the moment.
Summary: I picked this up to mainly shoot Bald Eagles. I have used this lense less than I thought I would, as it is very heavy and takes up a LOT of room on road trips. I have shot pictures of the moon using a 2x Canon extension and a Canon 7d which brings up the total mm to 2560. I can barely fit the moon in the frame of the picture. Who needs a telescope.