I have the Sigma 50mm 1.4. It's a very nice lens, especially for portait work on a crop frame. I love 50mm, and I use it a lot. I looked very carefully at the 50mm Canon offerings before purchasing the Sigma - after all, it costs more than the Canon equivalent. In the end, I decided that the Canon was optimised more for the landscape market that it was designed for back in the 1950s (the optical design is that old) and did not reflect what I do with 50mm today - looking for good bokeh and all that. And the Canon autofocus motor seemed a bit naff at this price. So I went for the Sigma, rightly or wrongly. But I'm happy with my decision. I also have the Canon 50mm f1.8 (mark 1 version - with the metal lens mount!) which costs the price a round of drinks, and is a great lens for more general work. Get one on eBay... but don't get the mark II - it has a crappy plastic lens mount and structure, which breaks in about 10 minutes, from friend's experience.
I think the 'Sigma lottery' refers to the possibility that Canon will change the electronic/software lens interface in some way in order to 'inadvertently' invalidate 3rd party lenses. I think that's actually very hard nowadays, because they have to maintain backward compatibility with their entire back catalogue of lenses back to 1987, but I guess theoretically it could happen. All I can say is that I've no compatibility issues with this lens whatsoever. And it autofocuses great by the way - fast and accurate, IMHO.
My only minor gripe is the size - its a little on the weighty side.
Review is at: http://www.dpreview.com/products/sigma/lenses/sigma_50_1p4