I've had one of these lenses for several years, now. Mine is the HSM version for Canon and was re-chipped by Sigma (I bought mine in the January sales back in 2004 when it was a new but 'discontinued' item and the total cost including the lens and all postage charges to and from Sigma, etc. was £270, which makes it the best '
value' wildlife lens in the world to me!). Re-chipping is no longer possible, so it's not a great deal of use without it - the main problem being the lack of communication with the lens aperture which is not possible to operate by any mechanical means on Canon models.
As Adrian says, it can be a bit slow to focus (not drastically so) and using the focus-limiter switch will help a great deal in this respect as a relatively large part of the focus travel is in the close-focus area. It's a fairly old(1995-ish) pre-digital design but there again, the Canon 400mm F5.6 design is older and that's still going strong. My mate has the Canon version and without doing a scientific side-by-side check, there's very little 'real-world' difference in the finished prints that we produce - except when it comes to butterflies and dragonflies, etc., where I can fill the frame without too much problem with the subject whereas my mate then has to either crop his image or change to his macro lens to compete. I should emphasise that I've been using either a 300mm or 400mm lens of some sort for about 40 years, often hand-held, so a bit of practice is needed to get consistently sharp results.
Check my gallery here on Birdforum:
http://www.birdforum.net/gallery/showgallery.php/cat/500/ppuser/1805 where a large percentage of the shots are taken with this lens. Many of the butterfly shots from my Continental holidays were only really possible with this lens as time would not have allowed me to concentrate on each subject with my 50mm macro lens (not without getting a serious talking-to from my ever-patient wife, anyway!).
This test report:
http://www.photozone.de/canon-eos/306-sigma-af-400mm-f56-hsm-apo-macro-test-report--review found the lens to be slightly better than the Canon model on an 8Mp EOS camera. As the report suggests the focus-limiter and AF/MF switches feel a bit flimsy but I've not had any problems - yet.