What's the niche? The one that's been asked for by photographers for years: a well constructed telephoto zoom that doesn't lose a stop of light at 200mm, that's much smaller and lighter than a 70-200mm 2.8, has high quality construction and very good optics. The Canon equivalent has been an extremely popular alternative to their f/2.8 lenses, and many Nikon photographers want an equivalent choice.
As for me, I own a 70-200mm f/2.8 VRII and have already ordered an f/4 version. Traveling with the f/2.8 version isn't terribly pleasant and this lens should be distinct improvement. I've known it was on its way for a while and am glad to see it arriving soon. I won't be getting rid of the f/2.8 lens as it's great in many situation, but this will be a nice alternative to the lower-end 70-300mm VR I've used while traveling.