I watched this about six months back. Liked it quite a bit, mostly for the art (what cool opening credits!), the setup, and the middle three episodes that had very little to do with the main plot. Seems like it suffered from the problem that isn't uncommon in anime where things are very mysterious: the explanation isn't worth all the build-up. I found it enjoyable almost all the way through, though, so I wasn't fussed by the not great explanation.
Free plug for a mysterious anime that doesn't dissapoint at any point: Haibane Renmei.