What do you guys think about a Sigma 17-50 f2.8 for an everyday lens for the Nikon D90? If I upgrade my camera, it will be to another DX lens. Just can't justify the heavier full frame and pricier lenses. Right now I have the Nikon 18-200 Vrii, Nikon 35mm f/1.8, Tokina 100mm f/2.8 (micro), and Nikon 40mm f/2.8 (micro). I may sell the Nikon 40mm but if I get this, maybe.