So you're wondering: how good is the Sigma 30mm prime? It's sharp, really sharp. Border performance will only be an issue when you shoot in a lab setting. Here are some samples from f/1.4 to f/5.6, click for mega size.
f/1.4:
f/2:
f/2.8:
f/4:
f/5.6:
How many lenses are sharp and contrasty wide open, at f1.4? Right. As you can see, this one is.
The verdict? You'll love this lens. It is built like a tank, fast-focusing, sharp, has lovely bokeh and a versatile focal length.
The bad? You'll have to find a good copy. A lot of these are back or front focusing. Test it before you buy it or make sure you can return it.
The alternatives?
For Canon: The 28mm f/1.8 USM lens is softer wide open, has more CA and doesn't come close to the Sigma's beautiful bokeh. It does however have more reliable and accurate focus. Another important advantage is that it has a smaller minimum focus distance. The Sigma's MFD is annoyingly far (0.4m), so keep that in mind.
For Nikon: The 35mm f/1.8 AF-S DX lens is a sharp and optically well performing lens. But, the bokeh quality is again on another level compared to the Sigma, it just doesn't have that creamy magic. Reasons to consider this lens are it's better MFD, smaller size and weight, and price. Reasons not to get it are the cheapish build quality and high CA's. Did I mention bokeh?
For both of these goes: they only go up to f/1.8. That's two-thirds of a stop difference compared to f1.4. More practically: shooting 1/15th of a second versus 1/25th. If you want to suck up all available light, Sigma is the way to go.
Questions? Send me a message: me at zerosignal dot nl