As far as Sigma, I have a 17-50mm f/2.8, and a 70-200mm f/2.8. Neither of these are Art lenses, but I've rented a few of them in the last year or so for weddings, and I have nothing but good things to say about them. I'm particularly fond of their 30mm f/1.8, which is made for a crop sensor, but it can still be used on a full-frame with a nice, creamy vignette at the corners. I have a specific project I want to use this lens for, in fact. ;-)