Is the gameplay pretty similar to Crisis Core? That's been my favorite (non-port) PSP game yet.
It's an action-RPG, so the game shares a few similarities with Crisis Core; and, like Crisis Core, the game was adapted well to the PSP's control scheme, so you won't spend much time wrestling with buttons.
The game has a required install, with the largest option being almost 900MB's of data; but it's so worth it, especially once you see how gorgeous it is and how quick the load times are.
I'm not going to try and explain the combat system, simply because I'd never do it justice. If I were you, just head over to GameTrailers.com and watch their review of the game. That said, I'm positive that if you enjoyed Crisis Core, you'll love Birth By Sleep. Love. It.