UPDATE,
We bought the Sigma 20 f1.8, Sigma 30 f1.4, Sigma 50mm f1.4, and 85mm f1.4 with Nikon mounts.
The 50mm is very impressive, as is the 85mm, yet to have a good test with the 20mm and the 30mm.
None of the these shift the front element when focusing, or extend the barrel, which makes life with the mattebox easier. I have been using the Zeiss ZP.1 glass recently, and while good glass, the breathing barrel drove me bonkers with the mattebox.
I have read reviews where there was concern with the Sigma’s sharpness above f2.8, and while this might be a concern with full frame 35mm, the glass is very clean on the 4/3rds sensor, and I have seen none of the vignetting or soft corners that were a concern with the reviews. This is obviously down to the 2x crop factor working in my favour for once!!
Bokeh is very nice, but the lens focus ring is a bit stiff compared to the Zeiss, a smaller .8 wheel on the follow focus should address this issue, though it’s not much of a concern.
The 20mm has a manual focus ring, while the rest are controlled with the Novoflex Nikon to 4/3rds adapter ring.