Sounds quite useful for sports and wildlife shooters.
Some of them, yes.
The problem is that the lens is slow and heavy and expensive compared with Canon's other offerings, and the zoom really doesn't get you all that much advantage.
Most situations where you're using a Great White, things are actually rather predictable. Sports in particular; the players are confined to the playing field and virtually always follow the rules of the game. You should know where the action will be before the play begins, and it should be very obvious where the action is moving to.
And there's much less need for a zoom at supertelephoto ranges. If a runner is coming straight at you from a hundred yards away, it's going to be a long time before her apparent size changes significantly -- more than enough time to sling the Great White over your shoulder and pick up your second body with the 70-200. But if she's coming straight at you from ten yards away, in the blink of an eye she's going to go from filling the frame with a 200mm lens to filling the frame of a 180° fisheye.
That's why the combination of the 70-200 plus a supertelephoto is so useful. Past 200 you can crop the 200 until the supertelephoto takes over; at that point, you really don't need a zoom any more.
What you might
need is more focal length, which really means more speed. And that's where this thing is desperately lacking. With the TC engaged, it's a 280-560 f/5.6. A 300mm f/5.6 lens is pathetic, and a 560mm f/5.6 lens is rather sad. Indeed, I'm having a hard time imagining this lens at 560 significantly beating the 400 f/2.8 II simply cropped to the same field of view. Even if it's better on the test bench...well, in the field, the 400 has a two stop advantage over the 200-400 in that situation, and that'll overwhelm any theoretical advantages the 200-400 might have.
And past that? The 400 turns into an 800 f/5.6 with a 2X TC with performance not that far off the actual 800 f/5.6, Canon's biggest and most expensive lens on the market. The 200-400 is going to need an external TC in addition to its built-in one, and then it's an 800 f/8 that probably needs to be stopped down to f/11 - f/13, hardly better than a RokiBowYang mirror telephoto.
Again, if it weighed five or six pounds (regardless of price) or if it cost five or six grand (regardless of weight) or if it was a stop faster (regardless of both price and weight), it'd be a very attractive lens, probably a game changer. But heavy and
three years late?