Suigetsu is alright. It seems like he's more for the sake of comic relief. But as far as characters go, Zabuza was a great one. Stronger than Kakashi, and played the part of the villain well, but even so he actually proved to be a good guy, with saving his enemies and holding back and wishing Haku had been given a better life. Compare him to Kisame, who really has no redeeming qualities and seems to be all talk, so yeah, he'll be tough to replace. Suigetsu does bring something new to the table though.
