I´m shooting in sports. The big advantage of this lens is the zoom-range with an aperture of 2,8. I often stand in a fixed position whith a lot of other photografers around me, unable to move and in this situation a zoom is really top. In such moments I use the EF 2,8 70-200 USM IS II with both tc (1,4 / 2,0).
The sigma lens would be very interesting if the iq is at the same as the canon-zoom -- especially when using a tc. In earlier times I tested serveral sigma zooms (the sigma 120-300 too) and the iq of the pure lens was ok, but when using a tc it becomes bad and no more usable and so it was no more interesting for me.
I´m hoping that the iq now ist better than all earlier versions especially when using a tc. Such a lens I would buy for 3500,- € and save 3000,- against the canon 2,8 300 IS II.
I`ll wait for the tests and images, then I will rent the lens and then I will see it the lens is worth the money. For a lens with only "good" image-quality I wouldn`t pay so much money. In such case I will save it and buy next year the canon superlens (2,8 / 300 II)
So, Sigma ! I hope you will give as a top lens