Imagine taking a PS2, with a gimped controller, and playing it in a car, with 6-8 hour (i believe, but not sure) battery life.
If you like the idea of that, then go buy one.
From what people have told me, you can play any PS1 game on your PS3 (or in the drive) on your PSP.
But obviously, I do not have one, and I've only played other peoples so I really can't contribute that much.