Completely spoiled! In my search for longer reach I've been researching the Tamron and Sigma 150-600 as they would fit within my price range. I was within a hair's breadth of opting for the Sigma but...............and this isn't to knock other people's choices...............I don't think that I'd ever be satisfied with another lens now that I have used this.
Once the proud owner of a hasselblad system, I had exactly the same problem 35yrs ago when switching back to 35mm and opted forever only to own primes.
Just before pressing that "buy" button on the sigma, I decided to search the forum on both the Tamron and the sigma putting more faith in the forum user's images/comments than on professional reviews. Glad I did! There's some excellent photographers and images on both, but the IQ bears no resemblance to the images that I have seen on this thread, and yes, I know that there's a huge difference in price between them, and canon L glass, and the tammy and sigma are a compromise, but I personally, just can't compromise IQ for better reach/cheaper price.
Once again, not knocking others' choices or the lenses, but simply trying to illustrate just how great I think this lens is and possibly, or should I say, more than likely, just how anal I am with regards to IQ
I shoot a few airshows per year with a friend of mine who uses a Nikon D810 with the Sigma 150-600 Contemporary. His images are very good! But under close scrutiny, they don't match my former 7D2 + 1.4x III + 100-400 II. My 1DX2 raised the bar even more. At the "bird's feather" level, the Sigma wouldn't do very well. IMHO, even at $2100 (US), the 100-400 II is the best bang for the buck that I've ever had from Canon.