It's pretty good. The controls take the usual hit from the PSP having less buttons and no second analog to move the camera, but it played alright. I was kind of used to it anyway from playing R&C: Size Matters (also made by High Impact Games). Flicking through the inventory screen it seems there's a shitload of collectibles and plenty of arbitrary challenges that award points used for buying secrets and such, which is always a bonus for a platformer. Basically it's exactly what I'd expect from a new J&D game, except it's on the PSP and not the PS3.
Not a fan of Jak's look though. Keira seems... odd, too. Don't know what it is. The character designs annoyed me a bit in Jak 2 as well. I much prefered the look of the first game, and the 3rd game did pull some of that back. This one is full on Jak 2 though, only with a crew cut.