Sigma, Tamron F2.8 Non-variable Zooms?

I notice that Sigma, Tamron and Tokina all offer modestly priced zooms from wide to short telephoto (24-60, 24-70, 28-75 etc) with non-variable f2.8 apertures. I know at a third of the price or less - these wouldn't stand up against the Nikon 28-70 F2.8, but I cant even seem to find an on-line review of them anywhere. Appreciate anyone sharing any knowledge of first hand experience with any of these.