I bought this lens only a few weeks back, used - but in perfect condition. The guy who sold it to me was using an old minolta dslr and was selling on his gear to go Nikon.
Anyway, the lens was mint, and worked no problem for 2 weeks. Until one day I switched to it while out shooting and it made a horrible creaking and whirring noise once I switched the camera on. The Af busted, for no reason whatsoever, MF works but the lens still creaks and moans when the cam is switched on and it's pretty much been left on the shelf since, bar the odd time MF was suitable.
A bit of research on the lens/Sony Alpha models revealed that this is a very common issue with this particular lens. The Drive on the Alpha body is too strong for the lens connections and pretty much breaks the AF on the lens.
Don't just take my word for it, have a google around before considering it. I'd hate anyone to buy one brand new and then experience this! Sigma have other , better lenses, that don't have this problem with Alphas, so I'm not anti-sigma here or anything. Just this lousy lens. It's a pity, because the IQ is pretty good [better than the kit lens at least] and it's a nice hefty weight lens which I like. I still use it in MF for when I don't require quick focusing, but on a zoom you really need AF.
Nikon D90: Tamron 17-50mm f/2.8, Vivitar 70-210mm f/3.5, Next up: Nikkor 85mm f/1.8D