The storyline is one of the best in RPGs, but due to it's age it's very skeletal. More like following a plotline, rather than a storyline... and if you want to get the most out of it, you'll want to look up information on the characters and events which was published outside of the game.
Some spoilers, but if you want a more detailed view of the storyline check out The PSII Manga
. It's very well done, pretty accurate, but it's only about 4 chapters for now. The author lost his notes and concept drawings a long time ago, but recently found them and expressed interest in restarting the project. Worth a look, since the storyline even in those four chapters is better than all of what PSU has presented us with thus far IMO.
Also, as said PSII is a VERY difficult game. If you can beat it, you've earned bragging rights. Especially if you somehow manage to all characters to lvl 50.