There are several people i encountered that firmly believe that if the lens on your camera is not a "true brand" name (i.e. Nikon or Canon only) or a "fast" lens which we know it comes with a price, it is not worth it. I have a Nikon D50 with a 28-90mm Sigma f/3.5-5.6 lens and personaly I think it works realy good and was looking into upgrading my inventory and get a 50-200mm (VR) from Nikon, but also see that Sigma has something similar. Is there really a big difference between a Nikon (or Canon) vs a Sigma or Tamron?