I've had this lens for 4 months now. Mind you three weeks of that it was in the shop getting rechipped (got one of the early models that did not function with Canon 40d servo).
I did some tests on it in the shop against the Canon 100-400 before buying and found the OS and AF speed to be very close, if not on par with the Canon lens (tested it on my old 20d body so did not notice the servo problem at the same time).
Once I got home it took me a couple of weeks to realise the servo problem. I kept it for a little bit longer because I was having so much fun with it (hummingbirds etc.). Generally I was really happy with the lens before I sent it away. It responded well in most conditions and could hand hold down to 1/200 at 500mm and still get decent shots. But true to the Sigma web announcement I started to notice the servo issue. (Note: Sigma's turn around time for correcting this problem was excellent. I live in rural prairies of Canada and I had the lens back in 3 weeks, it had to go out to Ontario I believe).
Now, wow, this thing flies with the 40d. In servo mode it tracks extremely well. I've locked onto many birds in flight and it stays with them as long as I can pan quickly enough. I think for those that say the AF is slow must have a poor copy because it is darn near the same as my 70-200 2.8 IS. As for hunting, I will admit in low light it does, but in my experience once it finds the target it stays with it.
I will agree that the picture quality may not by on par with Canon lenses. I have notice the high saturation and some loss of detail when shooting bright objects (like flowers). So this lens is probably not for the pros (but I'm sure you pros already have the much coveted 400, 500, and/or 600mm anyway). So donít expect to become a Nation Geographic photographer with this lens. Having said that though when you are shooting wildlife and more muted colours especially in the 100-400 range or at 500 when the subject is full frame the detail is excellent, just as good as my 70-200 with a 1.4x on it. And yes stopping it down does help but watch the light and ISO.
Build quality seems very good, I have already knocked it over once and had it come flying off the truck seat when I had to hit the brakes, still going strong (I know I should be paying attention to the road but it is migration time out here and I canít help but keep an eye out for the birds). Locking mechanism is a good feature to prevent the lens from accidentally extending in these times. It does creep back when you are shooting at steep angles but your hand is on the lens anyway so it is easy to correct or hold it in place with the focus ring. Iím a bigger guy so find the lens easy to carry around, hand hold and find it quite comfortable to use.
Overall I am having a ton of fun with this lens and getting some shots that I consider to be long term keepers. Letís see if Canon can keep pace by updating its 100-400, until then Iím happy. FM site only gives the option of whole numbers for overall rating so I gave it an 8 but would like to give it an 8.5. Cheers