TL;DR: I hope thy make a 35 and a 50. I'm probably biased, I've always liked Tokina lenses. They aren't the best but they're well made and enjoyable. This 20mm is a winner.
Only had a brief, windy, dark outing with it, but it performed stellar. It's nice to have a fast wide prime that isn't a massive adapted beast from a DSLR body. The Firin is quite compact and easy to work with. Performance seems good (in the dark) straight from wide open. I'll do more testing as I go, but this seems to be a winner, and as part of a 3 or 4 lens set (throw in a 75 or so) and it'd be all you need.
Update: Took the Firin out this afternoon for some more testing. I'm liking it more and more. Lens shows good separation at f/2 and closer distances. Central sharpness is simply stellar (the lizard's head was basically centered, then cropped). It has a pleasant vignette at f/2 that I personally prefer. If the lens didn't do it I'd probably add it in post. Bokeh is fairly smooth, but check out a few of the shots in the trees. I don't consider this too bad a performance for a 20mm. When there aren't points of light I find it pretty calm, other than the "coins" nothing jumped out at me.
Stopped down at longer distances the lens also sings. Same as last night, I find it hits infinity a bit before the hard stop. Stopped down it seems you can use the hard stop without much penalty though. Edge to edge performance works well for me. All of the below are edited, I'm not comparing lenses so I want to see how they'll do creating a final image. Disclaimer: I'm not saying this are the finest photos ever, it's what I could pull off on a lunch break.
Lastly, since we're all hopeless addicts, some central and edge crops from f/2 to f/22 for the below scene. Honestly I have to work to find significant differences. I mean they are there, but f/2 at infinity is great pretty far into the corner. I'll do some other testing later to better evaluate the corners, but I don't expect to be disappointed.
That's all for today. The sun was high, the light was harsh. Didn't get to check flare, but I think that's been covered already and it's a decent but not spectacular performer. Sunstars will be interesting too. The light last night suggests you want f/16, and they still aren't a Loxia, but I'm not as much a sunstar junkie as many, so I don't think it'll bother me. This one's staying in the bag a bit.