If I enjoyed Castle of Illusion, will I like Kingdom Hearts?

Only you can tell.

Didnt play Castle of Illusion, but Kingdom Hearts its a great saga!

A bit likely. They are totally different genres: KH is a true, 100% JRPG with most JRPG tropes all around. Not to mention that the Disney characters are just supporting cast.

kinda of... they are not comparable if you are talking about genres types.
if you are all open and just love the franchise, I'm pretty sure you'll love it.

As Talgaby said, it's a different genre.
Kingdom Hearts is a really, really good saga. If you like action RPGs, and if you are not disturbed by the presence of Disney worlds and characters in a fantasy game (that's not childish at all in the game), you'd love KH.
And if you begin with KH Birth by Sleep, the episode 0 of the saga, you'll begin with (for my part) the best episode of the saga.
I'm almost 25, I'm a man, and I'm in a hurry as hell to play KH3. (I'm waiting this for almost 10 years now...)