I just got my PSP with dissidia when it came out about two weeks ago. I haven't played Crisis Core yet, but I did buy it.
If you like Final Fantasy you will like Dissidia. I have put over 80 hours in it so far, and would have put in more if I didn't have finals going on right now. There are some flaws in the game, but the story is good and its really a neat mix of fighting and RPG elements.
So far I would say its worth it for the PSP, but remember its still new to me!