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I had the 31mm for a little while and now have the much cheaper (older withough HSM) Sigma 30mm 1.4.
First, the Sigma is APSC only, maybe it matters to you, maybe not. The 31mm is of course a FF compatible lens.
I haven't use the 31mm enough to give you a faire comparaison, so I won't do that. I'll however comment on sharpness and share some other random thoughts.
The Sigma is as sharp wide open as any f/1.4 lens I've seen. I will say however that I only use the lens wide open at relatively close range, for portraiture and similar application, so well within 8 meters. Beyond that, it's probably fine as well. Not that I would shoot at infinity and wide open. The center focus point is dead accurate on my K-5ii (I may have calibrated the lens, but that's something I do with all of my lenses anyway). I never noticed any problem with off center point either, but I mostly use center point. The focussing is about as fast a other screw drive lenses in that focal range, maybe a little slower than the DA 35mm 2.4, but nothing to complain about on my side.
As mentioned before, I use the Sigma for portraiture, low light indoor photojournalism and other indoor assignments. I never use this lens hoping to get edge to edge sharpness, so I wouldn't even be able to tell you how it fairs in that regard. Some people say corners and edge are terribles, I've never notice it, but my edges and corners are usually oof anyway.
About rendering, I absolutely love the shallow depth of field this lens provide and I haven't notice any problem in the bokeh either. I don't think it has a lot of character, meaning it seems "normal" most of time. To better explain, I'll try to compare with other lenses. The DA* 55mm 1.4 in comparaison sometimes offer Bokeh that will make you look twice and go "wait, what just happened?" because the Bokeh is either very nervous, or looks strange in some way. It's not a bad thing, but it happens. My 50mm f1.7 will also offer some strange, but lovely, Bokeh as well sometimes. It seems to me the Sigma is more constant in that regards, in the context I use it in anyway. I honestly think it doesn't offer anything crazy, but that it does offer very nice images for a good price and I'm perfectly happy with it. I will say however that I'm never tempted to use it outside in good light condition as I just don't think it would make the image pop as a limited like the 31mm would. But for indoor low light stuff, I think it rocks really good.
The FA 31mm offers a rendering that is more complex to me. I was looking at some shots I took wide open with it and couldn't help but look twice at most of the images. Most of them were snapshots of my kids doing their things, or picture in a book store, etc. I don't know if it was the colours, the contrast, or simply the presence of subtle chromatics aberrations in the transition from in focus to the out of focus zone that made the image special but truth be told, even after selling the lens, I could tell it produced some "special" images for me. Also, when I had it, I was more compelled to use outdoor - I think the coating made for better images of the outdoor colours.
I don't know what you would use the lens for (do tell us if you have an idea, it may help people offer you comparison), but I can recommend the Sigma for anything low light as I believe it offers good sharpness at 1.4 and pleasant bokeh, without being a complicated lens in my view, like some other lenses are. (For instance, I find hard to like the DA 70mm 2.4, but it sometimes just offer me that very nice picture that convince me to keep it, even if I own the 50-135mm that covers the 70mm focal length and is about as fast of a lens (although much bigger, of course!).
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Originally posted by NaClH2O 
Post #3 by kp0c
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