The Tamron 17-50mm f 2.8 is the Tamron ‘Professional Grade’ wide angle lens. It is designed to compete directly with the Nikon 17-55 f2.8 and provides similar range and sharpness. Overall a very affordable alternative, somewhat smaller and lighter than the Nikon lens. It is well reviewed on a number of sites.
Our opinion: The Nikon is probably a tiny bit sharper, but the Tamron’s very, very close, quite a bit lighter, and only 2/3’s of the price.