They cut the story short, introduce a smathering of new twists and "end" it up with a horrible DBZ-esque(sorry great Huddleston, my liege, I swear I'll never write that again) boss fight, which has, among other idiocies, two super beings yelling and powering themselves up. Which isn't really the point of Claymore. Or interesting.
The manga, on the other hand, is made of win and awesome. Pieta is a turning point, not the end. Well, it is an ending of sorts, but also a completely unexpected turn that has nothing to do with the main character getting a sudden power boost. Or finding a new ability. Or everyone being betrayed by a friend. Or any of the usual copouts.
Just read it, instead of a suffering through the Tour d'Offal that is the anime ending.