What I see in the show, is the great, great, GREAT animation and artwork of Yoshitoshi ABe. its not to show off what anime really is to people surrounded by Pokémon or dragonballZ...but to show off how well ABe can build characters and towns and an entire world with so many differences that it would astound the minds of "hit" movie watchers.
Its not mass marketed for everyone. I would relate it to movies like SLC punk, Eternal Sunshine of the Spotless Mind, Chungking Express, and Requiem for a Dream. They all try and challenge you to see the deep meanings behind the screen.
Music. Music can make or break a movie, and if it makes it, it either builds it up, or points it in a direction you didn't see before. The music in Haibane Renmei made it and it built up big time. The music gives it a deep RPG feeling, without the tedious and repetitive battles.
final opinion: not everyone will want to watch it, but having it in your collection is the anime version of having a 1830's bottle of wine in your wine cellar.
