Here is a recent comparison of 50mm lenses for Nikon F mount - obviously the Sony is not included. It shows some strengths and weaknesses of the Sigma. Warning - not a very good test but some of the comparison images are very enlightening.
I have the Sigma 50 for Nikon. Really love it for the way it renders defocus, to the point that I almost always use it wide open (with ND filter in sunlight).
The Sigma has its strengths and weaknesses. Let's start with weaknesses:
- Wide open, performance outside a 12mm radius is not so good. corners get smeared a bit, and for night shots there is significant coma. It's better at f/2, very good at f/2.8 and excellent at f/4. This is something to keep in mind when shooting at night - if you have to have street lighs in the corners then stepping down to f/2 will extend the great bokeh all the way into the corners.
- Like all lenses I have seen that are affordably priced, this lens is not apochromatic. Out of focus blown specular highlight/shadow transitions will sometimes show green/magenta edge casts.
- Internal focusing is always nice, makes for a sturdier construction. It isn't true IF though, rather the entire lens group moves inside the barrel. This means air is drawn into the camera body but the lens does not breathe air in between its elements. Keeping a filter on at all times will prevent the lens from drawing in outside air.
- Center circle resolution is great even wide open (with my sample).
- Fall-off wide open is less than any other 50 I have used or seen tested against it. See link above.
- Bokeh is to die for. It's absolutely magic. This is one of two lenses I have a emotional attachment to.
Build quality: it isn't metal, but it's the best composite construction I have used. I am not so worried about composite construction in primes, it's more of an issue with zooms. the solid barrel construction helps. Manual focus feel is decent for an AF lens. The included hood stays on nice and tight.
I was worried about the matte coating Sigma uses, that it would wear quickly, but so far (almost two years) it's handled field use pretty well.
Sigma has a reputation for inconsistent build quality. I cannot comment on that from experience as I only have one Sigma lens. Sigma seems to have had some problems with getting Canon EOS focusing compatible - no idea about Sony.
If defocus rendering is a priority for you (rather than general performance) then this is a lens for you. Rather than capturing reality, the Sigman creates beautiful images. I see it as a bit of a special lens, with a lot of character. Perhaps see it as a complement to a general purpose 50, partly because if its weight. For Nikon I think it complements a 50/1.8 well.