Forget any other Black Rock Shooter anime you might have seen. This couldn't be more different. It stars the same characters (ish, sort of), but with a different universe, premise and genre. "Reboot" isn't a strong enough word.
Imagine "Black Rock Shooter meets The Terminator", but in a cooler, weirder future with an Iron Ocean, a pillar that goes up to the moon and truck-sized killer cubes that drift around like jellyfish and kill you. The planet's been trashed and it's the year 2062. Mankind lost the war against the AIs and only a few of us are left now. Black Rock Shooter (aka. Empress), Strength and Dead Master are semi-immortal killing machines with impaired emotions and memories, roaming the planet and occasionally encountering a rapist robot called Smiley and his Educational Institute.
(Empress can transform her arm into an energy cannon that could blast through oil tankers, Dead Master is a sniper and Strength has artificial arms that must weigh several tons each. What's more, there's no real world. We're stuck with the girls' combat versions. It's as if someone decided to make a Black Rock Shooter anime set entirely in the Otherworld, but more explicitly post-apocalyptic. Also, Empress has a talking bike that swears in English.)
For a while, this is pretty boring. The characters are one-dimensional, no one likes anyone else and you don't care about the fights. Empress has the emotional depth of a sheet of cardboard and she's been rendered in CGI that gives her already expressionless face almost no mobility at all. You got more acting in Thunderbirds. (The only faces we see in full are those of the superhuman Hemitos girls, since the humans have to wear protective masks against the toxic atmosphere and the robots all wear masks because it's creepy.)
Frankly, I was only slogging through because I'd just watched all the other Black Rock Shooter anime and I'm bloody-minded. Gradually, though, we gain perspective and the show gets interesting.
There's a reason why Strength and Dead Master hate Empress and why they're both pushing the borders of sanity. (The show improves when they reluctantly team up, because at last we have character interactions instead of uninvolving action scenes with a robot-like girl.) Smiley and the Educational Institute are appalling and excellent villains, but they've got nothing on Lunatic. Her plan in ep.11 is HOLY SHIT. There's a human commander who used to have a wife and daughter. There's a load of junk called Andy who's the best character in the show, even if he isn't around for long. (He's in ep.7, which was a pretty good episode.)
There are family connections. (Dark ones.) There are ethical discussions. There's a last episode that trumps everything else in the series and kills 70% of life on Earth. It even has an emotional postscript, which I found surprisingly affecting.
This show is far, far better than it looks at first. Later episodes put the earlier ones in context and demonstrate that the show wasn't actually that empty and shallow. The CGI animation is never good, but all those masks usually let it get away with it. (Lunatic is never even remotely sexy, though, despite at times theoretically being extremely so.)
Dunno if I'll ever rewatch this, but I'd consider it. I quite admire this show and I think it's by far the most successful Black Rock Shooter anime to date. Just be warned that it feels thin and rubbish in the early episodes.