I've spent less on my DS and extensive games collection than it would cost me to buy a standard PSP and two games, yet despite buying it nearly a year ago its still used nearly every day, and importantly for a handheld...still works and isn't damaged in the slightest despite a few drops. Advance Wars, Mr Driller, Zoo Keeper, Kirby etc, on the face of it they don't stand a hope in hell against TOCA, Tony Hawk, Burnout etc but as handheld games they have (in my opinion at least) been much better value for money.
I tried to justify owning a PSP but it just isn't happening, the games aren't really suited to genuine handheld use, i.e. 15 minutes on the bus every day, out and about etc. Works great in the living room, but surely thats what a PS2 is for!