Lots of older Sigma lenses don't work on new bodies etc. And lenses like the Tamron 17-50 sometimes report focus errors etc. I really hate the fact that Nikon and Canon don't make their communications open, or just license it out for a fee.
the REALLY old ones yeah sure, but have heard of 1 case of communication issues recently to be honest.
What I have noticed with a lot of new Sigma/Tamron lenses is that they are increasing sharpness, especially in the center, but the contrast is still not often up to Nikon, and things like Bokeh quality and other subjective rendition quality are not as good.
This makes the lenses do well in objective tests but you can still see something special in Canon L glass and Nikon gold-ring.
again not always the case, example the sigma 50mm f1.4 in reviews of that class comes back certainly above the canon version.

