To illustrate my point, here are some sample photographs taken with the Sigma 30/1.4. Please note that this is not a scientific test, but I think it's a fair reflection of the lens performance as I see it.
Wide open (full picture resized). Even when scaled down, the purple fringing remains visible.
Wide open (left side detail - 100% size)
Wide open (upper right corner - 100% size). Image is quite soft in the corners.
F/5.6 (full picture resized)
F/5.6 (left side detail - 100% size). F/5.6 but still some purple fringing
F/5.6 (upper right corner - 100% size). Image is still not very sharp in the corner.
The Sigma 30/1.4 is not a bad lens: it has very good performance in the center of the frame and great bokeh. It is just not very well suited to my style of photographs.
Cheers!