well seeing as yuna was my main attack power, I didn't have a special need for kimahri, although he is one of the most interesting characters in the whole game, got lot of potential, but it was sad that he didn't have his own stuff that he could learn...

I think that with the way he was setup, plus his start position on the sphere grid, Kimahri is more of a "Make him what you will" type of character, which adds a nice twist to the game in my point of view.

he role is general distruction
he is good at the small ones and the large ones
he is good at magic emamys (ultama is in his area)
and his weapon is also persing so the def specilasts can be killed by him as well.

I did find Kimahri somewhat worthless, I only leveled him so i didn't have weak characters.
His OD was no help when battling the Dark Aeons and Arena monsters, Especially considering how good the ODs of Wakka and Tidus were and how useful Grand Summoning was in difficult battles, Hell even Rikku's mix helps during Nemesis and Auron's Banishing Blade helped with some high Def Bosses.

I didn't really find him all that useful, there was always another character who had better skills than him.

I found Kimahri pretty useful for the storyline gameplay, mighty guard was particularly useful on bosses. But end game wise hes not very useful. When it comes to fighting the battle arena originals, dark aeons and Penance, the obvious party choice is Tidus, Wakka and Yuna. Nonetheless, i did sometimes bring him out for mighty guard on some of the faster dark aeon fights.
So overall he's not totally worthless in battle, but definitely not a character to have as part of your main 3.