I am an amateur who has started photography as a hobby a few months ago. My observation is subjective based on my weekend photography expeditions and not based on any tests. I have a Canon EOS 600D Rebel T3i camera, a portrait lense, a canon 55-250 mm lense (not L class). Recently I bought Tamron SP AF 200-500 mm F/5-6.3 Di lens after a lot of deliberation, weighing this product against Sigma's 150-500 mm lens. In India, this Sigma is less expensive than the Tamron's product , but it is heavier and you might get tired hand holding it, but it has image stabilisation, unlike Tamron. But for capturing birds and wildlife in action, which I am interested in, not having image stabilisation did not pose any hurdle as i have to anyway use higher shutter speeds of 1/500 -1/1000. The image quality is very good. I am very pleased with the results. Will post some pictures soon. ( I have not used Canon L series lenses, particularly the 100-400 mm lens, which my friends say, gives very good images even when used by a half blind man. But you pay a higher price for it.) Tamron is a good buy. If you are in India and plan to buy, get it from somebody coming from the UW, where it costs much less--about $950. In India, the value added tax and other levies make it at least 50% more expensive. Vendors say you pay the price for a warranty applicable in India, which you may not get if you buy it in the US. I guess duty free shops at airports do give international warranty. It is worth trying. Also, it might be worth buying directly from Tamron's distributor in India rather than from a camera shop. Thanks.