I love this game. Admittedly, the difficulty ramps up slowly, but it didn't bother me. The tracks have so many branches and shortcuts, I just had fun exploring them. Once you get up in the ranks the races get plenty tough. Unlocking everything is a real challenge.
I actually enjoyed this game as much as the PS3 Motorstorm games. Though the graphics aren't as good (obviously), they're great for a PSP game. I'd put it up there with the Wipeout games as one of the best arcade racers on the system.