Since the Sigma 50-100mm f/1.8 Art lens was announced, I've been thinking about the correlation of this lens with the Olympus SHG ZD 35-100mm f/2.0 and the relationship from the Sigma 18-35mm f/1.8 to the Olympus SHG ZD 14-35mm f/2.0 lens.
It almost seems as though Sigma is doing this for bragging rights alone.
I have three of these lenses and I'm contemplating the Sigma 50-100mm f/1.8.
Mostly, I photograph at skate parks. That requires very responsive camera bodies and lenses. So far, the Nikon D7200 and Sigma 18-35mm f/1.8 lens combination has been better at wide-normal casual situations. Obviously, the lens isn't part of Sigma's Sport line but is part of their Art line.
The two Olympus lenses were part of their Super High Grade line, which meant that they could be used in any situations requiring the utmost quality and weather-sealing--and that they were very costly, somewhat above US$2000. I've used the 35-100mm f/2.0 lens for indoor high school sports such as swimming and basketball. The almost 3:1 zoom is 135 Format equivalent to 70-200mm, and not a bad range.
The trouble with the Sigma 50-100mm f/1.8 lens is that it has a limited range, similar to the 18-35mm f/1.8 lens. Occasionally, I switch the D7200 from DX coverage to 1.3x, to effectively extend the far end of the lens. It uses less of the sensor and therefore, less of the lens, so image quality should be improved at the edges.
Also, I find the 18-35mm f/1.8 lens to be very good, but not great. The Olympus 14-35mm f/2.0 on micro Four-Thirds equipment still works better for me, but the balance is off, even with the GH4. I wish that there was a tripod collar for the 14-35mm f/2.0 lens.
My experience with the 18-35mm f/1.8 not being incredibly good, I'm not enthusiastic about spending US$1099.99 on the 50-100mm f/1.8. It's not that I feel a need to buy it, but that it would make a good companion for the 18-35mm f/1.8. I suspect that, if I got a US$200-300 discount, I'd be much more likely to buy it, as I did with the 18-35mm f/1.8 lens.
I was watching a video espousing an opinion about the lens and it was somewhat amusing. I'd never seen that person's videos or web site, but the video was mistaken, at least, on the Nikon side about the possibilities for use of the 50-100mm f/1.8 lens.
At this point, I may not buy another lens for the D7200, although a normal zoom or the Nikkor 10.5mm f/2.8 fisheye lens could be useful. I'd really like to see the D7200 work well.