I was playing with the Nikon 17-55mm f/2.8 that a friend has and it seems like an amazing lense. The problem ofcourse is it's price (1200$). There is Sigma 18-50mm f/2.8 (420$), and Tamron 17-50mm f/2.8 (450$). Are they any good? Is the Nikon lense worth the difference? Dani.