There's some terrific stunt work in Ninja Assassin, but there's no "there" there. From my Fantastic Fest review:
"Contrary to a lot of the opinions I overheard at Fantastic Fest and that I've read since then, I don't think the action was too fast or too dark. I do think the movie could have been an amazing 20-minute short, however. The feature film that V for Vendetta director James McTeigue ended up with is one part bloody, violent fever dream and one part nostalgia for adults who grew up on the Teenage Mutant Ninja Turtles live action films of the 90's."
"Korean pop star-turned actor Rain (I'm A Cyborg, But That's OK) plays Raizo, a ninja assassin who goes rogue from his clan after they commit an unforgivable act of violence. Secret clans of ninja have apparently been killing people for centuries (accepting payment in gold), and Interpol is only now on their trail. The presence of various top grade actors like Randall Duk Kim, Rick Yune, and Ben Miles (best-known for Coupling, but he's done better) can't lift this one beyond being merely a great idea for a TV show from 1993."
"The action is ultra-bloody and rather intricate, but the story just isn't terribly engaging. I saw much better martial arts movies with admittedly thin plots at Fantastic Fest this year. They had a tiny fraction of the budget this one had, but delivered ten times the entertainment. Assassin will appeal to anyone looking for an adrenalized action alternative to holiday/Oscar season movies when it opens in November, but it's not destined to be terribly memorable to anyone."
The Blu-ray handles the dark scenes (most of the movie) really well. Blu-exclusive bonus features include a featurette all about The Myth and Legend of Ninjas, which dispels and informs, but meanders around far too long for my taste. Much more interesting was the ~10-minute and badly-titled The Extreme Sport of a Ninja, which touches on how much they relied on parkour practitioners to nail the balletic urban brawling they wanted.
Training Rain is primarily focused on covering how tremendously ripped the Korean pop star got for the movie. Also found on the DVD are 7 minutes of additional scenes that are mostly trims of existing stuff. The only silver lining I could find for people seeing this movie is that it results in their tracking down I'm a Cyborg, And That's OK, which also stars Rain and is far superior in every way (albeit entirely devoid of ninjas).