I haven't read the manga this dorama is based on, but considering that 99% of times Japanese adaptations from manga are very faithful (sometimes even too much for their own good, but that's another story), I bet the original was a real "page-turner"!
The premise is as catchy as they come, the constant twists and turns keep the viewer's interest Everest-high, all the superb cast does - bar one notable exception (*) - a great job (particular praises to Oguri Shun who's very good, to Shibasaki Kou who's a wonderful tsundere, and to the always great Nishida Toshiyuki), and the production level is absolutely top notch. I wasn't enthusiastic about the use of modern music at first (also, not being a fan of recent music - unless it's made to sound like it's older), but I have to admit it kinda works after all.
Honestly, I would've given this a 10, hadn't they decided to scr*w it all up by wrapping the story up lousily in a mostly poorly written, boringly diluted, obnoxiously hollywoodian movie. Without it, the dorama is great but feels unfinished; with it, it's half ruined. What a bad decision!

(*) Horibe Keisuke is plainly terrible as the Shogun (quoting myself from another review: "...compared to him, even Dick Dastardly from "Wacky Races" was a credible, realistic and sober villain!!! >__<")