If you want a portable PlayStation, the PSP is appropriately branded and does an ok job some of the time. If you're deep into the fighting games and you might have an itch for some tekken or guilty gear in the middle of the day, then it's pretty good.
If you're a little more into the 'dark side' stuff, the PSP has a thriving homebrew community. That's the main reason I got my PSP. There are some pretty nifty applications out there.

PSP is an ok portable with a few highlights that make it worth it depending on what your tastes and intentions are.

Just figured I'd make a positive PSP post, just this once. Anyone who knows me for 5 minutes can probably guess what I voted though.