I suppose, having watched as much anime as I have, I should stop wanting everything to make sense and happen for an explicit reason. Then I might be a little less 'WTF?!?' about this OVA. I enjoyed it a fair amount, I must say, though I found the slice-of-life portion to be fairly bland and the post-apocalyptic battle to be excllent and intriguing, but tragically unexplored.
Story: 7/10; good ideas, but much is left unexplored.
For a plot synopsis; two girls meet in a junior high school, become friends, and then drift apart. At the same time, two warriors resembling the schoolgirls are engaged in a battle to the death. The two parallel stories don't unfold on the same time scale; more than a year passes in the school, while all we see of the alternate universe is a single battle. The two parallel universes seem to be linked in some unexplored way, but why the two warriors are fighting and what the consequences of their fight are in the real world is left entirely unexplained. Maybe the creator wanted the audience to fill in the blanks themselves?
Art: 8/10; excellent fights and smooth action, but meh school scenes.
Unsurpisingly for an OVA, the art is excellent, especially in the post-apocalyptic battle. There's really only so much you can do with schoolgirls in fuku, though, and sometimes the real-world scenes seems like more of an afterthought to the dialogue between characters (which is mostly Lucky*Star type inane chatter, without the humor and otaku jokes).
Sound: 8/10; nice soundtrack, but some dialogue between our two fantasy girlies would've helped.
The music suited the mood well and I especially enjoyed the music that played throughout the battle sequences. The voice actors did well for what they were given, though in the battle story it mostly consisted of grunting without even calling out move names.
Character: 5/10; who is this devil girl and why is Rin's sister trying to kill her?
You'd think with two versions of each character, the main girls (Yomi and Mato) would be pretty well-developed characters. Not really, they're not really fleshed out much beyond basic character archetypes (sporty girl and quiet, mature girl), and we don't know anything whatsoever about our mysterious fighting girls, not even learning the protagonist's name until the very end. Still, I liked the character designs in the fantasy world, especially Yomi's succubus-like attire, so that's worth some points.
Enjoyment and Overall: 7/10; not sure what I just watched, but I liked it!
I wish more had been explained, but I've almost come to expect that from anime. They're not too great about filling in all the blanks, but perhaps I'm spoiled in that regard by American movies and television. The fight between the girls was killer; I'd have enjoyed it more if they had spent a lot less time on the slice-of-life BS, and more on what happened to the fantasy world and why BRS is fighting Devil Girl.