So I've owned the Sigma 135mm f/1.8 DG HSM Art lens for about 8 months now, however, it has been used sparingly for photography use. I have used it extensively with my C200 for wedding and other videography and it has been fantastic for this purposes (even if the auto focus isn't as accurate as the native Canon lenses with DPAF).
The Sigma 135mm is a beast of a lens. More than any other lens that I own (including the Canon EF 100-400mm f/4.5-5.6L IS II) it feels incredibly front-heavy on my 5D mkIV. As I usually handle video gear I never have any complaints about photography-only setups but I could understand how some people would object to this camera on the basis of its bulk and weight.
The lens itself presents very well. Aesthetically it is visually impressive. There are more external metal elements than any modern Canon lens you might come across (although nothing in comparison to the various lenses in the Zeiss range). The metal itself appears to be quality, however, it should be noted that the metal gets noticeably nicked over time. This is not a phenomena I've noticed with my Canon counterparts. The other gripe I have with the build quality is that when you turn the autofocus button on you can see a sliver of white behind the button. Though I can certainly live with it, this is somewhat disconcerting on an otherwise all-black lens.
Optically my copy of the lens exceeds my previous Canon 135mm f/2.0L. Even wide open this lens is often tack sharp (see images below).
I wouldn't bank on the autofocus as much as I would on my Canon lenses. Certainly I wouldn't be expecting performance like my Canon 16-35mm f/4.0L IS (which I have a focus retention rate of close to 100%). However, it's worth considering that this is a 135mm lens with an aperture of f/1.8. Such a feat is remarkable and should not be downplayed. If you insist on shooting on this lens wide open you'll have to be prepared to sift through your photos in Lightroom to identify the photos you've got in focus. As with any Sigma lens you have to be prepared to accept that there may be a degree of variance from copy to copy.
Finally, and probably most importantly, what do I think about the bokeh? Well, this is a pretty subjective assessment so be prepared to disagree with my findings. The Sigma 135mm does not have the most natural background blur out there. Undeniably the king is still the Canon 85mm f/1.2L II (of the current "modern" lenses). At times the Sigma 135mm has a nervousness to its bokeh that wouldn't be described as beautiful. I find that this largely affects out-of-focus items/people in the foreground of images. On the flip side if the bokeh is only really affecting a far-distant background it will provide a pleasant rendering of the out-of-focus elements and really allow the subject to pop out of the scenery they might be in.
To give you some examples of what I'm talking about I've posted some photos (and crops) below of photos shot on the 5D mkIV today with my 9 month old. In Australia it is the end of Autumn/start of Winter so I pushed the oranges particularly in post.
Overall Rating 8.5/10
Overall Image Quality 9/10
Value for Money 7/10