The HSM motor in the seems pretty fast especially for such a long lens. It's a hair slower than a Nikon 400mm f/2.8 with a 2X teleconverter, but the difference isn't significant. The low light conditions I normally shoot in cause almost any long lens to hunt, but the Sigma didnt seem any worse than other big lenses I've used.
You'll know by now that I'm a big fan of the Nikon 400mm f/2.8 lens, which coupled with a 2X teleconverter has been my primary long lens for a few years now. I shot the lenses side by side and the images below are 100% crops from the center of the image.
So can you guess which picture is from the Sigma and which is from the Nikon? The first one is from the Sigma and the second one's from the Nikon. You can make your own conclusions, but to my eyes the Nikon is quite a bit sharper. This couldn't have been due to lens shake as multiple shots were taken locked down on a tripod with 1/400 sec+ shutter speeds. Ignore the lighting differences as the sun went behind some clouds as I was shooting with the Nikon.
Stopping down seemed to help make the Sigma's output a little crisper, but not by much.
I've been asked a few times why my reviews of long lenses stick to analyzing just the center of the image in a field setting. Wouldn't shots of a test chart in perfect studio lighting conditions provide a much better guide to lens sharpness? I don't doubt it. I'm almost always shooting in bad light and less than ideal conditions, so performance in a studio setting means nothing to me. Also I hardly every put anything in the corners of the image so corners sharpness doesn't mean much either for a long lens.
Size & Weight
The picture below shows the Sigma 300-800 compared to the Nikon 400 (with 2X TC). They're both big lenses, but the Sigma is a few inches longer and a few pounds heavier. It's definitely enough to feel on a day in the field.
The Sigmonster is a very specialized tool for a specialized application. It's calling card is the sheer flexibility of the zooming. In my experience the greatest benefit is in zooming out to 300mm, finding the subject, and quickly zooming in to 800mm. Having missed great shots in the past due to being a split second late in finding the target, I can't stress enough how important this is.
On the downside, you do trade some size and sharpness disadvantages for this flexibility - some of the name brand prime lenses are noticeably sharper and weigh less to boot. Is this lens the right one for you? If you need the zooming flexibility in a super telephoto lens and don't mind carrying around a gargantuan lens, this might be the one!