I've got the 30mm f/1.4 EX DG HSM which I use with an Olympus E-600.
It provides great images with beautiful contrast, sometimes the results are gorgeous, limpid and well.. just beautiful.
However AF is very hit and miss for me. Sure the focusing motor is fast, but when it takes ages to decide wtf it wants to do that's academic. Slow in low light, but even in good light it sometimes misses. In fact when looking at the images it's hard to find what's actually in focus. I bought it for low light use without flash, but my f3.5 kit lens is actually more useful.
I have a feeling though that part of the issue is the incredibly narrow DoF at f1.4. A small amount of fore/aft shake could put the narrow plane of focus in mid air.
So, a bit of a love/hate thing. However for well-lit portraits it's outstanding.
P8235879.JPG by molgrips, on Flickr
Incidentally when flicking through the photos on flickr, I don't have to check the EXIF to see if they are Sigma 30mm photos - I can tell immediately