This is possibly the best shounen action manga I've read -- it borders on seinen because of the depth the series gains as it progresses. The characters are simply amazing. Each one of the Claymores is a unique warrior with a complex history. Part of the pleasure of reading this manga is discovering the stories behind the different Claymores. They aren't treated like women usually are by shounen and shoujo manga -- they're purer, in the sense that they're people, not mere gendered stereotypes. The women in this manga are real women -- well, except for the super powers, but you get what I'm saying. They are honestly and perfectly characterized, with neither their strength nor their vulnerability being exaggerated.
Not all of the Claymores are the noble warriors you may expect to meet after reading the first few chapters. Some of them are noble, sure, but some of them are indifferent, and others are malicious. Like any other group of human beings, they have flaws and the ability to overcome those flaws. What's rewarding is watching the flawed characters come to grips with their pasts and their mistakes, and try to improve themselves. It's a very universal and moving struggle. The Claymores' fight to retain their humanity isn't unlike the internal moral battles we non-manga people go through.
I totally agree with Laclongquan's review earlier. The manga's world opens up and grows more absorbing as you read it -- what seems at first to be a typical shounen plot gains incredible complexity without losing any of its coherency. Politics, power, ethics and of course personal angst are all brought to bear brilliantly. The mangaka has really done something amazing with what starts out as a simple monster-hunting concept.
My sincere thanks to the groups that are scanslating this manga for us. You guys rock.