A caveat that I saw the live action movies before I read the manga. (I believe the first movie is covered entirely in the first 3 volumes.) The story is quite different in those two mediums. I liked how they did the fiancee thing better in the movie, but there are also a lot of things I liked more in the manga... basically way more elaborate. The thinking behind this manga is just genius. I could just be a nerd, but I get so excited when L and Light reason and talk their way through stuff. It's deep, thoughtful stuff too, but despite it being talky at times, it never weighs down or slows the story (Liar Game has this problem). It's thrilling!!
So with so many great things to say, why a 9 only? If this manga ended in volume 7, it would have literally been an absolutely perfect manga that would be a clear 10/10. Gonna stop before I give anything else away. But even with Vol. 8-12, it's still a very solid read and one I would definitely recommend.
