I've been shooting with Sigma 28/1.8 EX DG on a850 for several months and find it quite a bargain lens for the performance it shows. It's not the lens everyone would call a SHARP one at any aperture, but very good in other respects like distortion, vignetting, CA, flare control, bokeh. It's hard to fully evaluate performance of a prime lens without comparing it to another equivalent prime. So I chose Zeiss 2/28 ZE, mounted it on Canon 6D and shot some comparison frames that you can find in this short review and download those DNGs for free.
I would like to hear opinions of those who have had experience with other fast (f/1.4-2.0) full frame 28mm lens and find them producing a better IQ in terms of sharpness, distortion, CA & bokeh at f/2.0 than aforementioned Sigma & Zeiss.
I know that Sony FE 28/2 comes out pretty soon and will give it a try before committing to a substitution decision.