A large part of what I do is sports or event related; for the big events I have a FF camera and my 70-200 f/2.8 IS II but there are also less critical events where I don't want to haul out the big system: due to the environment, saving wear and tear, lens choices, etc. I often use my 7D mk II which is an APS-C sensor. The big gap here is that I didn't have a zoom which compares to the 70-200 for this camera (and that lens is WAY too long on APS-C for the venue where it gets used). So for a while I had to make do with my 24-70 (short on the tele end) or my 24-105 (small aperture leading to slower/less accurate autofocus). The venue is very dark, with nasty old fluorescent lights. Even though I use strobes, the initial focus acquisition is troublesome. All my cameras, even the 1Dx, need something with at least a f/2.8 max aperture to focus consistently here. (Keep in mind that, especially using AI Servo as I have to, the more light reaching the AF sensor the better the camera can focus - even well above the specification limit. Additionally, in some cases, the larger aperture enables higher precision AF points for the high end Canon bodies)
For once, there isn't a Canon lens which meets my needs. I looked at the old Sigma 50-150, but the reviews were generally mediocre and it's hard to find. However, the new 50-100 looked like it might do the job. Reviews have been quite positive, the range looked like it might be adequate, and the unusually large max aperture promised better autofocus. I handled one recently on a 7D mk II and was encouraged by the feel and functionality, so ultimately I bought one.
This lens is big. And solid. It really feels like a quality piece of gear, and balances nicely on the 7D sized body. The controls are smooth and positive, and the rubber grips very easy to manipulate. The zoom ring is back near the camera body so that I don't accidentally touch the focus ring. And the lens includes a solid tripod mount. That's usually the first thing people complain about: that the mount gets in their way. Personally, if the subject hadn't been brought up in numerous reviews, I doubt I would have commented on it. I use the tripod mount on larger lenses like the 70-200 and 100-400 as a palm rest, giving my fingers freedom to manipulate the lens controls without abandoning the stable support. The same holds true for this lens; I don't have large hands, but I can easily turn the zoom ring with thumb and middle fingers while the mount rests on my palm and gives the lens stability. Not even an issue for me.
The only real problem I'm having in getting used to this lens is that the zoom ring turns the "wrong" way compared to every Canon lens. Some people dismiss this as just a preference and not a concern. Others, including me, have developed a methodology for framing moving objects that has been internalized - muscle memory if you will. I need to track moving subjects and anticipate where they will go, adjusting framing as they move. When I start out by turning the ring the wrong way and then having to reverse it, it forces me to hurry my compositions and make mistakes. Worse, not only do I change lenses during an event, but all except this one zoom in the direction I'm accustomed to. I put this on my camera last night and shot for 2 hours with just the one lens; even at the end of the event I needed to consciously remind myself that the lens zoomed the opposite way, which slowed down my response and made framing a frustrating task. Will I be able to train myself to use this lens properly? Yes, but it's an obstacle that I really wish I didn't have to deal with. Honestly, when using it I'll probably end up framing wider and cropping in post, which partially negates the advantage of a zoom.
So, how about autofocus? This was a pleasant surprise. I'm used to my 70-200 IS II which is a beast - it's so fast that sometimes I can't tell if it's focusing or not. The Sigma, with its huge max aperture, seemed every bit as good even in low light. I tried to make it fail by shooting through a smeared rainy window at a cable pedestal across the street. As long as it could see some contrast, it snapped into focus. Even when it had to hunt, the MFD to infinity traverse was impressively fast, though a bit noisier than the 70-200. Using it last night in poor light on subjects of varying contrast, it hit 99% of the time which is better than I expected. I'm very critical of autofocus, but this lens behaved quite well indeed and tracked moving subjects accurately. Incidentally, it isn't clear whether the high end Canon bodies enable their dual cross-type AF points for the Sigma large aperture lenses, and the camera data won't tell you whether it does. I have no quibbles with accuracy on this lens, though, so in the end it's a moot point. There are some indications that the lens ID sent by Sigma to the body indicates that this lens is a Canon 28mm f/1.8 (not the same as the EXIF, which correctly identifies the brand/model), and that this may in fact enable the high precision dual cross AF points. There isn't enough evidence to determine if this is actually true.
Image quality: to some, this is the only measurement that matters. To me, it's one of several; if I can't use the lens comfortably or get focus on the subject, it might as well be a pinhole camera for all the good image quality does me. I shot a few static subjects in poor light just to make use of the f/1.8 aperture, and the quality was excellent - even at the edges. My event last night was at f/4, just because I was using strobes and I needed enough depth of focus to include a small group of subjects, but it's where the lens will be used a lot so it was worth evaluating. I haven't really wrung it out yet with a variety of conditions, but initial results are everything I hoped for. It's a worthy challenger to my 70-200 IS II, and if you really need that narrow depth of field on APS-C, this lens will do it.
Now something a little controversial. I am unapologetic about my belief in AF microadjustment to get the best out of a lens. Expecting a manufacturer (let alone two manufacturers, such as in this case) to control tolerances to the level where the camera+lens is perfect is an unreasonable position. I will let this article speak for me, because Roger says it completely and eloquently. Sigma has taken some heat (from people who don't understand this) about their USB lens dock, which allows you to correct for these tolerances yourself instead of playing merry-go-round with the manufacturer(s) and sending equipment back until you get lucky. I prefer to put the lens on and get down to business as quickly as possible, which means configuring everything I can for my setup at home.
Because I've had a lot of experience making adjustments on my Canon/Canon systems, I was able to configure this lens, using the dock, fairly quickly (I'd say half an hour to mount/unmount it twice, measure the offsets, and update the lens settings). When done, a quick check of the camera/lens microadjustment (using the camera internal settings) showed that the AF calibration had been a success. After critiquing the photos from my event last night, I think one small tweak will make it near perfect. I may write an article on how to approach this, because there seems to be a lot of confusion about how to determine the lens offsets, especially on a Canon body. For me, it was painless and quick - but as I said, I've done this a lot in the past. I think Sigma should be commended for making a full lens calibration possible to end users who feel they need it, at a bargain price.
My overall impression of the 50-100 Art? An excellent lens, mechanically impressive and optically superb. I'm a little disappointed that I have to deal with the reversed zoom direction, but since everything else about this lens is first rate, I'm not going to downgrade my rating much because of it. If I can condition myself to compensate for it, the lens should work well for me.