I JUST replied in a thread where I brought up this lens and was asked about its quality. Here's my response:
I was very impressed with the build quality of the Sigma. It's the first Sigma I'd ever purchased, and I have to say if the rest of their higher end lenses share similar builds, I'd definitely purchase another. Focus/zoom rings are smooth, but not loose. The lens is very sturdy, and zooming is handled semi-internally -- the front element moves slightly, but does not lengthen the lens as it is a unified body. Overall, I'd say it's pretty darn close to L build quality... as close as I've seen from a third party, anyway.
Image quality is decent. It's sharp in the center, but a bit softer on the edges. This doesn't bother me a ton though, as edge softness is expected once you get into the Ultra-Wide realm, and this is mostly minimized once you stop down to f/8+. Lines remain very straight, with obvious distortion at the edges -- but again, this is typical of any UW. There's only very slight pincushioning at the center, but I've never felt it noticeable enough (if at all) to bother correcting. Chromatic aberration is the only thing that gives me pause when pulling this lens out of my bag. It's more than I'm used to out of my Canon lenses, but this very well could be due to the lack of in-camera lens correction for a non-Canon lens.
As for a filter thread, it kind of has one. The lens has a built in hood, with an attachment that covers the hood providing filter threads. However, I noticed some very slight, but tight vignetting even when attached to my 7D, so I haven't even bothered using it while on my 5Dm3. Also, using a polarizing filter on an UW can get you mixed results...
I'm also including one of my favorite shots with this lens. Looking up at the Manhattan facing tower of the Brooklyn Bridge last year. It's a three exposure HDR shot for contrast reasons, but even f/4.5 (wide open) it's still reasonably sharp on the edges. Oh, and this was on my 7D, so you can only imagine how wide it would be on a FF. :-)