I REALLY liked that OP. especially considering how much I had to rewatch it to keep track of the names.
Kekkai Sensen has really good music for its OP and its ED. Although my experience with anime is rather limited, I would say with conviction that its ED is my favorite, followed maybe by Roundabout or Koi No Shirushi. Mirai Nikki's first OP and ED were also pretty fun.
But yeah, Bitter Song to Sugar Step makes me dance on my seat every time.
EDIT: Here, the actual video:
EDIT: It doesn't work ._. Here's a link.