Dissidia's main storyline problem is its constrained up the ass, due to the fact it's on the PSP. You just can't get around that.
What it does give you (if you make it past the fact its been condensed worst than canned milk), is pretty interesting and amusing. The references to the past series, FF1, etc. They did their best to include lots of nods to the original series, along with the background of Chaos, Cosmos, and FF1. All of these issues hardly detracts from the actual meat of the game itself, which is the gameplay and customization. The shit's fun to play with your friends and creating unique artifacts is some of the best shit you can do to personalize your game. There's so much you can do with the characters once you go head on into the RPG mechanics.
If you're just grinding against the CPU then yeah. The game's going to get boring as all hell. The fun will disappear. But isn't the point of ANY fighter game, to play with your friends? The CPU is no substitute for actual, skilled, human opponents.