Well, I'm going to write here more or less what I wrote couple years back in Amazon as review for this lens and people didn't like to hear (for some reason, rave reviews usually get better ratings by other readers than heavily criticizing ones, but that's a topic for other forums) ...
I bought one just before I went on a trip to Canada, the lens was a HUGE disappointment (pun intended).
I already had a Nikkor 35mm f/2, and really wanted the wider f stop for better low available light photography, therefore I bought one.
I shot ~40 photos in a dim basilica, about half the photos with the
30mm f/1.4 lens and half with another Sigma lens: the
10-20mm wide zoom (all on a
Nikon D70 body, which I love), Almost all photos taken with the 30mm came out out of focus, almost all photos taken with the wide zoom came out nice and sharp.
Already discussed this on other forums and folks suggested that large aparture lenses are prone to focus errors while wide open. Well... ins't the whole point of this sigma lens the larger aparture!? the Nikkor 35mm f/2 costs about the same, weighs half, is half the size, and far better optically at apertures 2.0 or higher, whats the point of getting the Sigma if it isn't the f<2.0 numbers? Right, it has an internal focusing motor which the Nikkor does not have. A plus for
D40 owners.
I admit I never got to the bottom if this, my feeling had always been that I just received a defective unit, and, since by the time I discovered this I was far away from the store, I never had the time and patience to ask and arrange for a replacement. On the other hand if you google for reviewes on this lens carefully, I believe you'll find I am not the only one to complian about focusing problems.
Two years after, the Sigma 30mm f/1.4 collects dust in the cupboard, the Sigma 10-20mm, Nikkor 35mm f/2.0, and the Nikkor 18-70mm (the original Kit lens) are always with me.
For me, the Sigma 30mm f/1.4 was one of those "great on paper but mediocre in real life" products.