I originally bought this lens on a whim a few months after getting my 6D. The POTN Sample Photo Thread had enough stunning images that this college intern needed to buy it. This lens lives on my 6D and is my absolute favorite. All in all, I highly recommend this lens. The sample photo thread speaks for itself.
This lens is wicked sharp wide open, and gets sharper as I stop it down. I do minimal sharpening and am always happy with results.
The autofocus on this lens is a dream. I'd read stories about how Sigma wasn't known for autofocus, it was hit and miss, etc, but that's not the case with my 35. Autofocus is really fast, and with the centerpoint on my 6D, I can focus in some really dark settings. It is my fastest focusing lens - it is faster than my 70-200 f/4.
Chromatic Aberration / Coma / Distortion / Vignetting
I don't notice much chromatic aberration with this lens, and anything that is there gets cleaned up nicely by the defaults in Lightroom. Coma is very minimal. I go back and forth with this lens and my 24 Art for astrophotography. This lens has much better corner performance than the 24 Art, but is isn't as wide, which is a fun trade-off to make. I don't notice much distortion with this lens that isn't easily corrected with the Lightroom defaults. It does vignette, but I find it to be a very pleasing amount, and don't often correct it.
I've used this lens with my Hoya R72 IR Filter and the lens does have an IR hotspot in the center. Here is a forum post I made about it.