Like others, I have drooled for a long time for this copy, and now it's here and I am glad I have picked up this one. Almost all potential buyers' setback of this lens is its price tag, but after owning it, using it, and examined it in details, I think it's worthed in every fraction of a penny that I have spent, and I am sure the ones that have already own it are with me.
I am not getting it for shooting sports, at least that's what this lens is known for, but rather for wildlife and nature. I have been seeking advices from a lot of wildlife Pros and got a lot of advices from others as well, before acquiring this lens. Most would recommended the EF 500mm for it's reach and reasonable price tag, and a few also mentioned the EF 600mm. It has not been easy to make the decision of which lens to get, but the f/2.8 offering by this lens has attracted me the most. Most personal reviews have said it's "fast fast fast! sharp sharp sharp!", and I don't need to repeat that. I have been tracking in-flight birds at the lake with it and the 1D MkII, in low light, wide open and it works flawlessly. Most have told me since the latest DSLR's can handle higher ISO so well, that the f/2.8 is not necessary. But I find wide-open shootings offer more controls, such as enable the camera to shoot in as low as ISO 50 at dim light, and/or freeze the frame at higher ISO, yes, even in dim lighting! The reach, personally I start at 400mm, adding TC's 1.4x and 2x as needed. I find shooting with the TC 1.4x it is as fast and as sharp, little or no quality/light lost (wide-open at f/4). Shooting it with the TC 2x (wide-open at f/5.6) could hurt a little in sharpness, but could be regained later in Photoshop, contrast is also lower, but to trade these with 800mm at f/5.6? I'll do it! All controls/functions in the lens work perfectly.
Supporting it in a way to provide stability while shooting is another big thing. I have not tested anything before buying but have known about the full Wimberley Head, and I have chosen it with the Gitzo G1325 tripod (this is as well the most common set-up by most big lens shooters) to support the EOS-1D Mark II, EF 400mm f/2.8L IS USM, RRS's flash bracket, the 580EX with the flash X-Tender (Better Beamer), all well over 15 lbs, weightless! It is actually "fun" swinging this big set-up all-over and still shooting at a rate of...what?...8.5 fps. Some thought the Wimberley Sidekick alone can do the same job, I could only believe it. I have tried to use a monopod to support this lens but I could not handle it too well, mainly because it's still a bit too heavy for me.
I hope I have not written too much, it's just because it's hard to find in-depth information about this lens anywhere, and it seemed most reviewers/readers only interested in with the latest digital camera bodies only, so I rather be writing a little more than needed, than just little information. Please note all these are my personal thoughts of this lens, others may not be agreed with me, and you have to try one for yourself to know. Pros and Cons: same as others have said here. Goodluck!