Definitely wildlife photographers. I'd go for such a lens if I didn't already have the 200/2.8 + TCs. That said, it would be a nice complement, especially if it really is a constant aperture.
Bear in mind that Nikon offer the 200-500 which on the D500 (for which it is really designed) is 300-750, so the Olympus zoom makes sense. And wildlife shooters love that lens.
I really wondered if Olympus were down and out with the changes in the relationship from a marriage to friends with benefits with Panasonic but with the emphasis of these new products on the sports, wildlife, action and dare I say it, reportage market segment perhaps they are cleverly finding a new niche?
LouisB