It is important to know before starting Monster that this is a manga for mature readers. There are no chibi characters, no comic relief moments when things get too serious. The art is very realistic.
I read the manga after I had already watched the anime. Even though I knew most of the stories and the ending, I can say that Monster is a truly remarkable manga. It is one of the greatest psychological thrillers for more mature readers. It keeps you on your toes often. It is interesting and spooky and almost supernatural at times. The stories of many characters are developed very well (some even too well, considering some characters are not very important to the story).
The only reason I am not giving it a 10, is that the ending is a bit anti-climactic, compared to the big buildup towards the end.