http://en.wikipedia.org/wiki/Sound_of_the_Sky
If you have not watched it then I suggest you do. It ponders life, its meaning or lack thereof, friendship, and the bond between people. It was a lot of fun.
Plus, hearing a certain song played in Sora no Woto made me want to learn how to play trumptu~.

I watched and I loved it too, despite the lack of story development (especially at the end...)
The music was very good, the chara design and the voices nice too.
But what I prefered in this anime was the post apocalyptic background, I was a little bit nostalgic :)

I'm a year late to the party, but I watched this recently and it was brilliant.
The Slice-of-Life style was pretty nice, but the way it turns that idea completely on its head surprised me. The whole series was constantly edgy, but in the best way.
The only problem is finding others who have watched it...
Oh, and I loved the soundtrack.