i only have one "off brand" lens (a old old series 1 vivitar 70-300 af) and even though it's a tank and i never really use it, i've never had any issues with it. but that's all the experience i've had. tons of people seem to use sigma, so they must be pretty darn good.
it's an allright lens, not the fastest thing, i got it for my f100 35mm set up so i could have something wide, I'd save up the money and get the tokina 12-24, that thing is amazing and has become my primary lens, and with a constant f.4 who could complain!?