The reason I got rid of my PSP is, despite having some great games with killer graphics, it just doesn't work well as a portable system. Sure, it's handheld, but it's too large to take anywhere; the screen can't be protected like the DS; the load times are way too long to justify playing in short spurts; and the battery charge length is short (4-5 hours on average).
It's still a great system, but it doesn't have what I think a portable system should -- long battery length, quick load times, games that are fun to play in short spurts or long bursts, easy to protect and stick in your pocket, and flat out fun.