I thought he had a load of power...I mean one of his punches was enough to knock out Chiriku in a heart beat. I think between all of Hidan's raging and Kakuzu wanting to kill him we didn't get to see his fullest strategic potential.
Hidan: Hey Kakuzu - what do you call that?
Kakuzu: It's a Water Clone.
Hidan: @#^#@!?
Kakuzu had Kakashi, Ino, and Choji all on their toes. His movable hearts did some tough damage...if Yamato and Naruto hadn't come in time Kakashi would have been gone.
I'd say he's one of the top 3 badass members following Kisame and Pain.