They're not really 'notorious' for focusing problems, but they do seem to get a bad rap.
This would be the 'thirdparty fear factor' .. where thirdparty lenses will be under closer scrutiny, than those lenses of the genuine manufacturer, would be.
I think a lot of social issues could be explained due to this phenomenon.
I think there is a situation where many genuine manufacturer lenses will also be having 'focus' issues, but the users simply don't understand it correctly enough.
Look up the term
'focus shift' and how it affects fast prime lenses. Very few, if any, will not be affected by the problem.. of course except manual focus lenses, like the 50/1.2
If you have a D300 or better/later camera body with focus micro adjust, this feature is ideal to determine if the lens has any 'focus issues'. If it does, you can take it to a local Sigma repairer to get it adjusted properly.
Also note! Apparently the local distributor for Sigma, has this deal where they price match any grey import price. look for prices according to a local grey product, get in touch with Sigma's distributor and see how ya go!.. you then get an official product for the same price and no warranty issues.
ps. mine is over a year old now and still going strong!