So right now i have the Sigma 18-50mm F/2.8 EX DC and am fairly happy with the lens. I've been debating buying the Nikon 17-55/2.8 and found a good deal on a gently used one. The problem is after reading some reviews it seems the Nikon lens does not always get the best reviews considering it's much heftier price tag. Anybody here got any real work experience between the two?