A lot of lens reviewers don't test lenses in more challenging real world situations (dark and artificially lit rooms, low ambient light etc), and these are the places where people are reporting the most issues. Showing how sharp a lens is under controlled studio tests is also to be taken with a massive pinch of salt!
I'm not sure it's a case of newbies finding the shallow DOF of a fast lens too hard to handle either, as I've seen lots of people who have said they had used the 35L for years, were ready to switch to the Sigma because of its surperior optics, but ultimately didn't due to its inconsistencies. If it was just one or two I'd be dubious, but there are too many people reporting the same issues for it simply to be false rumours from clueless people, IMHO.
Ultimately, it's the OP's choice, just thought he should be aware of what others have reported. I'd suggest that if you do go with the Sigma then make sure you buy from a reputable seller who will be easy to return it to if you do have issues, and make sure you test the hell out of it whilst still within the returns period!