I don't think Haas ever intended to write a masterpiece; just an honest piece of music for chamber orchestra. It just so happens that it was pretty well-received; I heard the conductor Simon Rattle, among some others, sing its praises, and decided that I should listen myself. I particularly liked the way in which a gradually shifting tempo can create a sort of fractal effect, so you don't know which sound-layer is which anymore.
I had never heard that before, and was very much struck when I finally did. The closest thing I can think of is Ligeti's "devil-staircase" effect, which we hear in the piano etudes; but even this is an entirely different idea. As far as Grisey is concerned: yes, Haas seems very much influenced by the French spectral music movement. But it's a relationship similar to the one between Beethoven and Brahms; the influence is notable, but both composers were great in their own right. And the idea of music enhancing natural resonance is part of our own zeitgeist. There's nothing wrong with that!
Maybe I'm naively optimistic, but I don't think anyone puts themselves at the top of any pyramid, and I think everybody writes music that's interesting in the same way that everyone is unique (whether I personally happen to like that piece is a completely different issue). But look at it this way: if I listen to Bach or Beethoven with the wrong mindset, it'll sound like cheesy, fancy, boring classical music. And if I listen to Webern or Boulez with the wrong mindset, it'll sound like weird, splashy, inaccessible, modernist crap. I happen to hold all four of those composers in high esteem, so I make sure to listen with the right mindset. When I do, the music is as interesting and intriguing as if I were listening for the first time.