I had a PSP and three games, which I traded in and got 4 DS games and a PS2 game instead.
My reasons are numerous, but the biggest ones would have to be that the load times are horrible and the controls are often clunky, as most games are not made for the PSP, but are made for the PS2 and then ported to the PSP.
You mentioned Me & My Katamari, but honestly, don't even bother. Just play the PS2 Katamari games. They control better on the PS2, the load times aren't as bad on the PS2, and the music is better for the PS2 versions. In the PSP Katamari, they just recycle the music from the previous games, and there aren't as many stages, either.
On the good side, there's finally some original content coming out on the PSP that's built specifically for it. Ultimate Ghosts N Goblins and LocoRoco, for example. And I think Namco is releasing a new Tales game, too.
I think I will eventually get a PSP again, but for now I'd rather put the money towards more DS games.