You have to judge on a lens by lens basis really. The Sigma 50mm 1.4 for example is a suberb lens. Likewise the Tamron 17-50 2.8.
Mainly a walk about lens.
I hired a canon 17-55 2.8, but I can not afford one of those, so I am thinking Tamron 17-50 2.8?
Experience over the years has taught me that Tamron QC seems to be consistent, Sigma's isn't.
Tamron apparently so I heard supply the parts for some of the Nikkors, it wouldn't surprise me.
I can put up with non USM AF when the lens is as good and sharp as the 17-50 F2.8, especially at its price point!
the sigma 18-50 f2.8 i used to have was an excellent lens