Plot was good, the mangaka just didn't know how to develop it.
So it slowly began to become worse and worse.
Last arc was a mess and was the worst arc of Claymore imho.
I'll tell you why it was a mess past vol 12 :
Power-up and power-up and power-up (exactly like dragon balls and narutos), asspulls, unnecessary subplots, poor dialogues, poorly written characters (isley), deus ex machinas (Riful 2.0 and Theresa), , abyss feeders (who thought that was a good idea ?), lack of plot depth (this is why I think the mangaka didn't know how to develop a story, he hinted a war past the island they were in but nothing) and Claire, the main character, being boring till the end.
The story was promising, full of mysteries, nice tone, nice fights, no over the top transformations, a darker shounen, I wanted it to challenge Berserk but it didn't... I loved it and it ended up getting me angry, chapters after chapters, such a wasted potential