It's a great manga of the action/fantasy genre.
It's a manga that take place in medieval times that enforces the idea that in order to defeat a monster you need to become one. Emphasizes the importance of love, teamwork and friendship. Good amount of drama and plot twists. In spite of the initial setting it's a really complex manga.
It's packed with action and has amazing plot. Even though it might seem pretty generic and basic at first, the art and action keeps you going until you reach the point where things start to get real and get entirely sucked in.
Out of all the manga I've read as a 1 decade long manga enthusiast.. for me Claymore is number one... BY FAR.