*Very* useful, but as mentioned it does depend a lot on your photographic style. A long lens is typically used to bring a subject closer (good for wild animals) but they can also be used to adjust the size relationship between subjects.
If you like the "telephoto look" (which comes from the relative subject distances and not the lens itself) then get this lens. If you want to take photos of wild animals (and don't want to spend a lot of $$$) then get this lens. On the other hand, if wide landscapes are your thing then this may not be the lens for you.