I have had the exact opposite experience. I have 2 Sigmas lense for my canon and they were very sharp right from the box. All manufactures produce a bad copy from time to time.
Err, I didn't say they weren't sharp. In fact both were very sharp, even (surprisingly) wide open. However, they had trouble with the autofocus. In fact, this is a very common problem with Sigma lenses as I found out after I started researching my issues. Just a take a quick look at some Amazon reviews (eg for the very sharp Sigma 28mm f/1.8 [amazon.com]
This isn't a problem of "a bad copy time to time", this is a consistent quality issue. These kinds of complaints are non-existent for Canon glass in this price range (eg the excellent Canon 50mm f/1.4 [amazon.com]