Hmmh, that's a toughie.
I've had Kingdom Hearts for a while, and haven't yet completed it (which isn't that surprising). I'm pretty much like you; if the game is too difficult, it'll cause me a lot of annoyance, and a degree of frustration.
Kingdom Hearts has had a few moments like that, so far. And they were all in/during big foozle battles. None were anything I'd start ripping my hair off, but nevertheless, there you are. Still, I overcame.
Now, the biggest problem I personally had with the game, was that all the fighting began to feel very monotonous. True, in action RPGs, it's all about slashing, slashing, and slashing. But, you're always rewarded with a lot of "bling bling" (ie. Dark Alliance), or unique new locations to explore (Beyond Good & Evil). And, although you do get your rewards (miniscule as they may be), I never felt they were enough to keep me wanting to fight these critters.
What's more, said critters rarely changed much. You'll always face those same types of buggers, to a greater or lesser extent. Or at least that's how it felt to me. This resulted into an overwhelming desire to just run right past everything, and not even bother with the fighting. And while it's something you certainly can do, and even should at some points, you'll still need the experience, or the big bad bosses will be a lot more annoying than you'd like.
On the other hand, the locals that the game offers, really are something else, and as you said, the production values are quite up there (as seen in graphics, music, sound effects, and good voice acting). The whole thing'll feel a bit strange in the beginning (teaming up with Pluto and Donald, of all things), but soon you'll get used to its rather unique take on... er... Final Fantasy meets Disney.
And most importantly, you'll find a lot more of a story in this Action RPG, than you would at most others, even if it is a tiny bit hokey.
So, should you spend $20 on it? Sure, give it a go. You may end up loving the game, and be glad to know there's a sequel in the near future waiting.