I have heard mention of the Sony screw drive not playing nicely with Sigma, especially the A900, but the fact remains that Sigma's also fail on other camera bodies as well, probably just as much.
However, lenses and bodies fail, it happens to every manufacturer, but what is inexcusable is taking a couple of months to fix a lens that was clearly defective, which I've also read about Sigma, who takes really long to replace or repair damaged glass (even though they have the best warranty around, at least on paper, 5 yrs in some cases).
I'm not bagging on Sigma. If you live close to a Sigma certified reseller or repair center, or you don't rely on a particular lens in a significant way, then getting a Sigma might actually be pretty good. However, if you live in the minnows like I do, I wouldn't want to take the chance and in the event that something does go wrong, be out in the cold for 6 months or more.
Granted, an equivalent Sony or Zeiss lens usually costs 2x as much, and although I am certain that some of that is just brand name inflation, I also think that a large portion of the higher price simple goes into better materials and better QC/QA systems. If you watch some of the unboxing videos for Zeiss, you see that each lens comes with a signed inspection card. That means someone sat with the lens, and made sure it was working correctly before it ever got sold. That is a reassuring feeling that just might justify the higher costs.
That being said, thousands of people use and swear by Sigma, and have never had a single problem and what I've said doesn't dismiss that (or attempts to). Overall, just make sure YOU understand what compromises you are making when you decide to buy a Sigma, Tamron, Sony or Zeiss lens.
@Heikki Taskinen: Maybe if you try to stop down the aperture a little that might fix the problem? Front focus, as far as I understand is rare (but it happens) and only serious testing under consistent and known situations can confirm its presence.
@popo: Sorry to hear about the lens. Is getting rid of the screw drive a good thing though? I mean if all lenses had a motor inside, they could still break and would probably cost quite a bit more. I dunno, would like to hear your opinion on this.