Context: Lens released 2012. I shoot mostly portraits, weddings, and some landscapes. I used the Canon mount version.
This is a cool lens! It's lightweight compared to most Canon L lenses, and is an excellent less-expensive alternative to something like the 35mm f1.4 L II. I love the look of this lens, and the manual focus ring is excellent. My three complaints: the numbers in the focus window could be much larger; the lack of weather sealing, which is the main downside to the Sigma Art lenses; and focus speed (more on that below).
However, I have only found the lack of a weather seal to be an issue in the most extreme circumstances. I was video-taping a thunderstorm with the lens on the camera, on a tripod, and fully exposed to the weather - that was the only time I have actually got the lens mount wet before. Next time I record a thunderstorm, I'll use a lens with weather sealing.
That being said, the overall design of this lens is pretty awesome, the switches on the side are great, and the three different subtile black finishes on the various parts of this lens are intriguing.
This lens focuses accurately for me, but is not as fast to focus as a Canon L. The 17-40 f4.0 L, for example, which I used to have, was much faster for focusing than this lens (the 17-40 and other Canon wide angle lenses have legendary focus speed though).
This is the highlight of this lens. The images are very sharp and chromatic aberration is minimal. The first time I took photos with this lens and then opened them on my computer I actually laughed out loud because the photo blew me away with its sharp subject in focus and nicely blurred out-of-focus areas.
But more important than sharpness, and the reason I sold my 17-40 f4 L to get this one - I can use this lens for portraits. For me, this lens provides a great wide-angle portrait lens, I don't have to be concerned about distortion (big noses, less flattering, etc.), unless I get in quite close. This is a great lens for small, dark venues, where you have people and want to keep everything looking pretty good and not distorted, while shooting handheld in low light with not much room to step backward.
In my opinion, this is a great value lens and I use it almost every day because of its wide aperture, image quality, and great 35mm perspective. However, the much older Canon 35mm f1.4 L is comparable in most ways and likely focuses faster. The newer 35mm 1.4 L II is an amazing lens that I can't afford. As far as I have seen, the image quality of the L II is even better than the Sigma, plus it has weather sealing and focuses much faster - but the price of the L II is ridiculous for a 35mm... especially in these uncertain economic conditions.
Gallery - Sigma 35mm Art lens
Gallery - Sigma 35mm Art lens
This lens is very useable at f1.4, and I often use it there for longer portraits. For landscapes, I use it at f8, where it's sharp all the way into the corners of the photo.