I'm replacing the CAnon 100-400 IS USM (original version) with this lens. The things that attracted me to the Sigma are the focal length, the all black body (I kind of hate the white body of L glass), and the updated technology (better OS, for example).Upon getting this lens, I noticed the box was huge. I was hoping the lens wouldn't be as big as the box made it seem. I was wrong. This lens is enormous. It's something so big that my large camera backpack may not fit it. It will be hard to take this out on casual hikes like I did my 100-400 (see the photo comparison of the size). And it will be really hard to handhold for long. I better hit the gym.On the styling front, the all black style is very sleek. The only thing I don't like about the design is the lens cap and hood. The lens "cap" is actually fabric, and it slides over the front of the lens. This is not quick to put on and off, so I will likely be leaving it off more than on.And the lens hood, besides also being enormous, is all metal. While that is nice from a protection point of view, it does make the lens heavier. And the last thing this lens needs is to be heavy.But enough about the size and design of the lens, how does it perform. I have taken the lens out a number of times, mostly in morning light, but also a bit indoors in the evening. The results have been about what I expected for image quality, and better than I expected for image stabilization.The Image quality can be seen in the included photos. Even wide open this is producing nice results. Stopping down to f/8 improves image quality a bit. Going to f/11 keeps the quality on par. Anything beyond that shouldn't really ever be used.But the image stabilization, wow! I have shot 1/40s at 600mm, and had sharp images. That is a solid 4 stops slower than the 1/600s handheld guideline for the focal length. I haven't yet played around with adjusting (using the Sigma dock) the IS, but I have heard that using Dynamic IS makes it even better.I have also found image focus to be good, although I plan on testing that and making adjustments if needed with the Dock. So far, things seem sharp, but it feels like they could be ever so sharper. It feels like the lens is backfocusing just a little bit when at 600mm. Luckily, the Sigma dock allows for a ton of adjustment here. You can not only set micro focus adjustments at the long end and short end of the zoom range, but also a couple stops in between. And you can also set the micro adjustments for different object lengths, in case you need a different adjustment for close objects vs far away objects. This process is likely going to take me an hour or two, but it should make the lens perform even better than it does out of the box.Focus has been pretty fast for me, even in a few tough situations. There are adjustments for this too in the dock. You can set it to fast, but less accurate. Slower but more accurate, or a balance. I have so far shot at the balanced setting, but I plan on trying out the slower but more accurate setting as well (I pref accuracy over quick focus 99% of the time).Finally, a couple of things to note. Back to the size/weight, I have found the tripod collar cannot be removed. If I were doing only handheld shooting (which is actually most of the time) I would like to remove that to save on weight. If I get out an allen wrench, I could remove the bottom portion, but then I have to keep track of the screws. Also, the front element is very heavy, so there is a lens lock, but my version only locks at the marked focal lengths. I hear the new version of the lens locks at any length, which would be nice in some situations.Overall, I'm pleased with this purchase.