*Prepares to get flamed*
In a lot of ways, COM was actually better than KH2. Why?
1. Better Org Characters. They actually did something interesting and weren't cliche'. Axel seemed a lot "cooler" in COM.
2. More difficult and complex battle system. I mean, you have to arrange cards in a strategy with attack, item, magic, and summon cards, instead of just mashing the triangle and x button.
3. More difficult boss battles. Hell, Larxene was the second to last Organization member to fight, and she took forever. Luxord and Saix left nothing to the imagination. Axel was also hard in COM, but soooo easy in KH2. And, the first battle and second battle with Marluxia were much harder than the Xemnas fights.
4. More than one controllable character. Face it, Roxas had the same basic attacks as Sora, but Riku could do lots of things. He also had a reverse storyline, and it was fun to do it during the time Sora was doing his own thing.
~AND~
5. More abilities. If you found all the sleights possible in COM, then you got more abilities than KH2. They were also more difficult to do and more fun in my opinion.
So, was COM worth it? I say yes, yes, yes! It kept the fans waiting for KH2, and it actually turned out to be a very high quality GBA game. In ratios, it had better quality than KH2. If COM was remade and put onto the PS2, it would be more fun than KH2, I can guarantee.