Most everyone that is reviewing lens has been giving it very good reviews. No, it's not going to perform as well as Canon's 200-400 f/4L IS 1.4x, but that lens costs about 10 times more!
I get asked a lot about what lens to buy, and one of the most asked is what super telephoto zoom to buy. Since the announcement and reviews of the Tamron 150-600 f/5-6.3 VC, I have had a hard time recommending Canon's long-in-the-tooth 100-400 f/4-5.6L IS. This is a great starting point for anyone wondering if or how they'd use the super telephoto focal lengths. Some people may find they sit in the 300mm range, others may find they never leave 600mm. If you spent a year with this Tamron, you may then have a better idea as to whether or not investing in one of the big Canon L's is the right decision for your situation and pocketbook.
I think it's very interesting watching Sigma and Tamron release lenses that are priced extremely competitively and give wonderful bang-for-the-buck performance. Your move Canon….
“I simply don’t have the budget to purchase many of the super-teles that cover this focal range, and furthermore, I don’t shoot this style of photography often enough to justify the expense even if I did. But this lens hits a sweet spot for me. It’s price is low enough to not only be affordable but also a small enough investment that you won’t feel like you have wasted your money if you aren’t shooting long distance every day. So if you can’t afford a “super-tele”, how about an “ultra-tele” (that’s what Tamron is calling this lens segment)? The preorder price in the United States is only $1069. It is about $180 more here in Canada, but this lens represents such a tremendous value that I have had a serious conversation with a friend in the industry about how it is even possible for Tamron to make a profit at this price. I personally wonder if they are not selling this lens at a loss to drive brand recognition. It’s that good.”