Listen. All the Kimahri hate makes me feel sad. I love the boye.
Just speaking for myself, I don't hate him at all. I love his design, and I like his personal story.
The problem for me is, that, like others mentioned, I never saw a way to make him into something specifically "him". He sort of has to become a copy of one of your other characters, and here I fail. I just don't like to have two times the same character. Like, I'm horribly bad at strategy games, because, instead of building stuff the way it makes sense, I want to have one of every building first. Was that way in Starcraft, was that way in Heroes of Might and Magic 3. One building after the other, I need everything. No doubles. So, Kimahri never had a chance. A shame - as I mentioned, I love his design. Ronso look so cool.
Also, the sphere grid is just intimidating to me. Every time I start the game, I take a look, and try to think of something interesting, but there is just too much stuff there. So I let everyone go their predestined way (except for Yuna, last time I gave her black magic, which works really well). Kimahri doesn't have one, and I just can't make a second character the same as another one. So, I just have nowhere to go with him.
I will try to use him, whenever I get to FF X.