I've always been a fan of anime series, my favourite being Berserk. I tend to focus a lot on the pacing and the built tension when I watch anime, and Berserk is The master of pacing. Another thing I tend to focus on is character development and overall sentimentality of the show. Many anime characters are quite over-the-top, a major let-down for me. As are many scenes in anime. Claymore succeeds on most levels. The pacing is some of the greatest I've ever seen, and the inclusion of ranked characters just adds to the excitement. It's always interesting to see who's got the higher rank. Furthermore, you really start caring for the characters and picking out your favorites (mine being no. 6, Miria). The show only fails on a few levels. You're in for a lot of melodramatic scenes, and the protagonist's male friend, Rocky, is just another annoying brat who cries more than good is. It's no worse than so many other animes and gets better halfway through. The second, and final, major letdown is the anti-climatic ending. There is an epic battle, but the way it ends after the battle is simply not good enough. It feels unfinished, ill and way too sentimental and moral for a show of this kind (it might have something to do with the continuing issues of the manga).
However, you're in for a treat. And you're not going to want to put this show down, before it's ended.