So much has been said about this lens already, but there are some things that I can share that I have not seen in previous posts. I use this lens with either a 1D Mk II or a 1D Mark IV body.
1. Unlike most of the reviewers I use this lens exclusively with a monopod/ballhead combo. I use a Giottos MH3300 ballhead, on top of a Manfrotto 681B Monopod. The main purpose of the ballhead is to provide a strong and stable platform for the lens to sit atop the monopod. I don't ever change the angle of or loosen the ballhead, rather I loosen the lens in the collar and rotate the lens/body to change the angle of the picture if I want to shoot vertically.
Even in high wind conditions with inherent instability (I shoot a lot of ocean sports), I get pin sharp results with this rig. It is a heavy rig, but not unbearable; it is nowhere near as heavy as trying to move a tripod around, and gives you complete flexibility in terms of moving around at will to get the shot you want.
I am able to take this rig into positions that would be unthinkable with a tripod, or without support, and get the exact shot I want.
The ballhead/monopod combo is extreme weight-rated to handle this load.
2. I am constantly amazed at the sharpness of the images this lens produces on a pro-body. Nothing else in my bag comes close. If I can use this lens under any circumstances, I will use it, even if it means lugging the load around all day.
3. At times I use it with a 1.4x extender but I find that this will produce usable pictures only in the best possible light - like on a bright sunny day. Shooting fast sports like surfing with the extender on a dull day is a waste of time IMO.
4. If you need distance/getting in close to the subject, then I would recommend you stick with a cropped sensor on a body such as a Mark IV with this lens. Testing this lens on a friends 1Dx, I could see no ability to compensate for the loss of magnification, which is very noticeable between cropped and full frame.
5. My biggest complaint with this lens, is that it appears to be hyper sensitive to micro-scratches... those tiny almost imperceptible swirls. I would love to only have to clean this lens once a year, but working with it in extreme environments, I have to clean it regularly. Slowly over time, no matter how careful I am, and believe me I treat this lens literally with kid gloves, it picks up these tiny whirls from the cleaning. They don't show up in the images, but they cause me real heart-burn looking at them!
6. The lens cost me a small fortune, but it produces a picture that is quite unique, which in turn leads to more sales, so it has been a very worthwhile investment.
I highly recommend it.