i bought a used version of the non-DG version of this lens and have been shooting birds and wildlife with it. before this, i was using a Nikkor 300/4 AF-S with Nikkor TC-14E II and TC-17E II extenders.
the 300/4 is an exceptionally sharp lens and is considerably ahead of the 800/5.6 in sharpness. however, with careful post processing with a good sharpening filter, it's possible to come close, certainly good enough for high quality 8x10 prints nearly indistinguishable from shots taken with the 300/4. at 11x14, there is a larger difference, but after post-processing, the results from the 800/5.6 are more than acceptable.
the 800/5.6 isn't as contrasty as my other Nikkor lenses. post processing can remove most of the difference, but images taken with it appear slightly flat compared to ones from my other lenses. this can be seen in side-by-side comparisons, but otherwise isn't noticeable.
the DOF on the 800/5.6 is extremely thin. even at 50 or 80 feet, a duck facing sideways will be entirely in focus, but a duck facing the camera will have at least one part of the body noticeably out of focus. larger birds can be worse.
AF speed is quite good on a Nikon D2-series body. it is able to keep up with larger flying birds at a distance and smaller birds that flit from branch to branch. using the focus limiter helps if the shooting range doesn't change much. it's not as fast as the 300/4 with the 1.4 extender attached, but the difference is small.
i have tried using a Kenko Pro 300 1.4 DG extender with the 800/5.6 and AF is inconsistent to non-existent even in very bright conditions. it's best to manual focus using an extender. the resulting 1120mm/8 lens doesn't deliver especially sharp results. it is better than just cropping and enlarging by 1.4X when printing, but not by much. the extra magnification also magnifies vibration blur.
the lens, like all Sigma lenses i have owned, has strange bokeh. it's not as obvious or as objectionable as i have seen in other Sigma lenses, but it is not a smooth as Nikkor or Pentax lenses that i own. the very limited DOF helps because if the bird is in the air or on a branch, the background is completely blurred. it's only when objects are near to the plane of focus that the bad bokeh shows up most.
at just over 10lb, the 800/5.6 is fairly light as far as ultra long telephoto lenses go, but even so, you will need a very sturdy support. i use a Wimberley head on a Gitzo 1325 and it is about as light as i recommend going. a 1548 would not be out of place supporting this lens.
the Sigma 800/5.6 APO EX HSM represents a good value in a super telephoto but at a noticeable, although acceptable, compromise in quality. much of this can be made up by careful post processing, but not all. it offers a good alternative to the manufacturer's offerings where cost is extremely important. it's the only AF 800/5.6 on the market and that counts for something.