I have the 80-400 OS DG version and using it with the 350D (XT). The optics are good and the image quality what I was expecting from the lens.
I did not notice any issues with the OS. What I did notice is that the lens causes a delay in rapid shooting (3fps) sometimes. I was able to determine that the symptom occurs when the shutter speed is between 1/125 ~ 1/160.
I cannot really understand why this happens, just an inconvenience. The end result is that shooting speed is reduced by a little, perhaps down to 2.5fps, however this is not constant. You can hear that shooting is not steady. Some frames shot at 3fps some at a very lower speed. Like stutter.
I have read some suggestions that it may have to do with too much current requirements from the lens side, and although this lens drains batteries like a ****, I am using it with a battery grip with two batteries and have seen no difference in performance. Just batteries getting drained fast.
The lens is not HSM and I have not read any rumours or plans to put HSM to it. The focusing speed is, for me, the biggest issue with this lens. AF is noisy and slow. In some cases AF will fail and at least the focus ring can be used in AF mode to focus manually.
I have trained myself in a technique for AF, by which I zoom out, focus, zoom in with the shutter button half pressed, and then take the shot.
Considering that with moving subjects AI Servo is the proper way to AF, this technique just blends with the AI Servo technique and is not really bothersome.
One nice thing with the lens is that it gives acceptable to good results with the Sigma 2xTC. Manual focus only, and it is a **** to focus, especially in close ranges, due to the very small depth of field. I need to stop down 1-2 stops to get very good results, and given that with the TC the min apperture becomes f/11, good pictures can be obtained at f/22 with the 2xTC attached. This really reduces the exposure / ISO options dramatically, which means that with the TC, I need to set ISO to 800 to take pictures.
On the brightest side, the OS works very well. At 400mm it is very hard trying to frame and focus without OS enabled. At 800mm, it becomes a nightmare. The OS works wonders there and allows the lens to be handholded at 800mm with very crisp results, provided ISO is set at 800 and sometimes the lens is used wide open.
This is clearly a bright sun lens.
Before choosing this lens, I was also considering the Canon 100-400. I did not select the Canon and I regret it. It doesn't cost much more and lacks nothing that the Sigma provides. The push-pull zoom is blamed by several people to be a source for dust suction, however the large tube on the Sigma is also sucking air when zoomed as well. I can hear the air flowing in and out while zooming.
All in all, this is no better lens than the Canon, is not substantially cheaper, so I cannot see any reason for choosing this over the Canon.
Since Nikon also has a similar lens, with a similar price, I can imagine this lens is not produced in numbers, hence very hard to find.
On the other hand, now that I have it, I find no reason to sell it and buy the Canon in it's place. I will, though, be investigating primes and brighter than the Sigma. The problem is, what I am looking for is called EF 300/2.8 and costs a lot.