New to the forums here. Registered just to respond to this question. I am not a technical guru but I can respond based on personal experience.
Simply put, I love the Sigma. I've used one for over 5 years and have graduated from the first version to the DO version and now have the newest with OS. As crazy as it may sound, it is my primary portrait lens, and I couldn't be happier with the results. Seriously, over 90% of the images in this gallery http://www.timdoolinphotography.com/p550601605
were captured with the lens. Without resorting to pixel-peeping, I'll stack the printed results of the lens against anything. The reach allows me to limit depth of field substantially despite the lens' slower speed. Using the lens allows me to photograph spontaneous behaviors of little children, brides and grooms, family members, wildlife, etc., very unobtrusively. The focal range allows me to do most of my portrait work without needing lens changes to add a tele-. I could go on and on about what I like about the lens. If you get a good copy, the sharpness is fantastic. Some copies are not as good. My very first version of the lens was soft, so I sent it back. The most recent 2 have been wonderful.
There are considerations when looking at the lens. It's heavy. At nearly 4.5 lbs, when attached to a pro grade body or one with grip attached, it's quite stout. I'm a big guy, so it's not an issue, but many would struggle holding steady without a tripod. It's also quite long when fully extended and with lens hood attached.
It is slower than many others. For my purposes, and with the advancements in low noise, higher ISO bodies, I'm getting great results. I especially love the new OS version for help in lower light.
It does not have as many seals and o-rings as the Canons, but I have been impressed with how little dust has appeared over the years. I've had mine cleaned once over that time.
The exterior rubberized coating is very nice to handle, making for a grippy surface, even when cold. It does begin to wear in frequently handled places more so than the white paint of the L line.
I'm beginning to ramble, but if you have any specific questions I haven't answered, let me know and I'll try to help out. For the money, the extended reach and the optical quality, I wouldn't be without one.