Iv'e been wanting to purchase a Canon telephoto lens either the 300 or 400 2.8 but I just cant bring myself to fork out that much money. I would if it was significantly better... Does anyone have any experience on Tamron or Sigmas compared to the Canons? How much better is the Canon than the Sigma or Tamrons. Also the IS feature on canons, A) Is there a feature like that on Tamrons or Sigmas. B) Does it really help that much? Is it a necessity?