Haibane Renmei has a subtle, mysterious charm to it. It has a very subdued atmosphere for the most part save for a few dramatic moments that play very well in contrast with and against the overall tone (if that makes sense).
Some things may not make sense to people though. I myself feel I may have missed some things in subtext, but I was still able to appreciate the overall themes and metaphors used throughout the anime. Rakka and Reki also lead the story along well, though no one else in the cast particularly stands out.
-minor spoilers below-
The Haibane Renmei is never fully explained and neither are the circumstances surrounding the premise such as why there are Haibane to begin with, what exactly makes the Toga so special (though I believe their true nature was implied along with the Communicator's), but some could argue that the vagueness fits well. As for me, I'm left wanting to know some more, but not desperately so that it knocks my overall enjoyment of the story.
-Ok, you can read again-
Overall, Haibane Renmei is a solid series that works well with its premise and for what it is, though it definitely isn't for everyone. Some might be put off by its slow pacing (even I'm guilty with switching between tabs a little more than I should have) and the lack of attention-seeking drives like action or comedy to pull the viewer in. But if you can look past that, I'd say give this a watch.