I have a Sigma 30 1.4. I tested two side by side, and they were both good. I wanted a fast general purpose prime and for crop, this lens is perfect.
I use it very rarely. Performance is good, but the razor thin depth of field you get at f/1.4 make many low light subjects very difficult. I often prefer to use flash instead. Unless of course, you want that shallow focus, in which case it's unbeatable.
The way I test new lenses is very simple, but is good at weeding out 'bad copies'. Shoot a distant subject at f/1.4. High shutter speed, high ISO. Focus very carefully, and shoot a target like a car number plate. Lock focus and exposure. Position the target in the corner of the frame and take a shot, then reposition it in the other three corners and take three more shots.
Then blow up each corner target on the LCD and compare them. They should all be equally sharp (or more likely, equally unsharp - this is a tough test). If one or more of the targets looks distinctly softer than the others, chances are the lens is not properly centred - which is the most common manufacturing fault. Just remember that you are not looking for perfect sharpness, becuase you won't find it with any lens with this test. You are looking for equality. And then just to show what the lens can do, shoot another frame at f/5.6 with the target in the centre. That will look pretty sharp