Why have people not heard about this show? I really don't get it. It has excellent characters, plot, and speed... really, it's just great on the whole.
Characters
The main villain (who I happen to think is FANTASTIC!) is probably one of the most intimidating characters I have ever seen. His mere presence is enough to get the entire cast quaking in their boots, and he just has to be there. Let me reiterate that: just his being there is enough to keep you tense for the next dozen episodes, even if he doesn't appear for the rest of that time. He is also very unique, in all aspects of his creation, appearance, background, and motivations.
The main character, who is voiced by the same person who voiced Gaara from Naruto, is brilliant. It's as simple as that. He is unique in every way, and extremely good as a hero. His own inner turmoil is something that kept me on the edge of my seat from start to finish.
Speaking of inner turmoil, many characters have some kind of insanity or paranoia plaguing them throughout the story. You would think this would get old, but let me assure you, IT DOES NOT! Their mental stress allows for some very intense and truly awesome scenes, all of which are very realistic and plausible. As one last note, the show skips over every cliche in the book. Almost every member of the cast is at least 25 years old, and those that aren't don't act as if they're 20, like in many anime. For characters, Monster gets a 10!
Art
I have to say, art wise, Monster takes a hit. It has a very realistic style compared to many shows, and for me, that's a bit of a turn off. Also, since it was made in 2005, the animation itself is a bit lackluster. However, after about episode 6 or so, you will hardly notice the small things that are done perhaps incorrectly, namely a few scenes where characters run. The way they swing their arms... eh, I don't know. However, once your past that point, the art is actually quite enjoyable. So, for art, Monster gets an 8!
Sound
The sound for Monster is good at all times, though in no way memorable. The musicians wanted to play music to enhance the mood, and that's what they did. No more, no less. A generous 8 for sound.
Story
The story is incredible. Imagine taking Death Note, removing all of the bad stuff and cliches, then put the good guys on the road. This is what Monster adds up to more or less, minus the actual death note. It's a crime story, chock full of psychologically powerful moments. The ideas and plot twists are numerous, and the story covers a sprawling area. There are few shows that can match the plot and scale of Monster.
The basic premise is somewhat simple: famous neurosurgeon saves child from a head-shot wound, child turns out to be a peculiar kind of sociopath who then proceeds to pin every crime on the man who saved him. The neurosurgeon, a man named Kenzo Tenma, is forced to hit the road on the run from the police. His only hope is to find the boy whose life he saved, and bring him to justice.
Like I said, this is just the basic premise. This is one of the most detailed shows I've ever seen, and side-plots and sub-plots are everywhere, all working in perfect unison to offer an incredible experience. Easily a 10.
Enjoyment
I thoroughly enjoyed every aspect of Monster, until it began to drag out around episode 65. The feeling soon passed, but the memory of it kept this show from the 10. A 9 that probably deserves a 10.
Overall
On the whole, I would call Monster a masterpiece. It's a true shame that it's so unheard of, when it could serve as the crown jewel of many an otaku's anime repertoire. It is my personal 5th favorite, out of (currently) 43 shows. A solid 9.5. Watch this show; I guarantee you, you won't regret it.
