So, I happened to see this the other night. I will say that, if nothing else, it's inventive and certainly highlights some of the best things about Japanese animation.
The story, though? Not really my cup of tea. It was weird, incredibly fast-paced and there were so many "twists" that after a while I wasn't surprised and, actually, was kind of bored.
I won't get it for my collection because I'll never watch it again (along with Gotham Knight but I have that one so...).
Overall grade: D
Watch at your own risk.
I give it better than a D, especially if one has a fondness for anime.
At first, i thought Batman: Ninja was an Elseworlds type story, but it's actually a time travel/screwed up history tale. It leans into the anime early on, then it just goes nuts and embraces it completely.
It's not going to be for everyone, but that short summary above might help you decide if it's right for you.