Monster is a very well executed anime, full of suspense and action, even though the action is kinda slow-paced itself.
The story is one of the best elements concerning Monster. Dr. Tenma was an amazing brain surgeon, but in just one moment his life turned upside down, from a genius doctor to a fugitive-avenger. With two opposite ideals clashing, that of love and equality against the one of a "perfect world" and life only for those who want to live, it shows how humans can become anything, even monsters with no emotion, as well as the power of love and friendship.
The animation is very good, and that's what most people say, but to me it seemed a bit too dim or, perhaps, too simple? I just wasn't really excited over the animation...
The sound was also very good, it totally fits together with the series, but it's a bit too simple and somehow monotonous!
Finally, the characters are superb, you just get stuck with them. Some of them you get to like and cheer for, and others make you hate them! But each character is special in his own way! They seem as true and existing as it gets!
In general, Monster is a pretty repetitive series that can even get a bit boring and slow-paced, but gradually you'll notice you can't get off your seat! It's a true suspense anime, which I definetely recommend to those who like mystery and humans with fearsome potential and abilities (those who can hardly be called humans)! And as you get closer to the end, you'll discover the quality of this extraordinary story, that of a human whom us, his fellow people, changed into a monster!