This replaces, to me, a 70-200/2, 8 IS. The reason is that the 70-200 is too cumbersome and it is all too often that the lens stays at home when it should be in a camera bag. Due its weight and size, you should ideally use a tripod, and it only adds to the feeling that it is cumbersome to take with and manage.
200/2, 8 halves the weight and size. It is much easier to stuff in a camera bag, and you do not need a tripod for it.
The quality is, of course, top class with great image quality at maximum aperture, solid build quality and reasonably fast autofocus.
The only thing it lacks is the image stabilization. In addition, it would have been nice with the weather seal on a lens of this caliber. On the other hand hurts not as much in the wallet with a price that is less than half of what 70-200/2, 8 IS costs.