If you have a psp and a pretty large memory stick, or lots of money to use on good games, and you have many long trips ahead of you, then yes, it's worth it..
My psp before I got a large memory stick, was about the same as my membership at the gym, unused.. Now, I *cough*dled*cough* a few games from different places (all legal, of course), and when I get to try all the games I want, then it's fun...
But the psp in general is really just a waste of money...