This one is my favorite since GTA 3
I personally liked the mafia story line of GTA 3 more than either the 1980's Miami Vice theme of Vice City and the 1990's "Boyz in the Hood" theme of San Andreas, so I was really glad to see they took the PSP installment back to Liberty City.
Just like the others the missions are fun, interactive, and have an "open-ended" nature to them. The goals are clearly defined but you can achive them, sometimes, in different ways.
The game has recieved some much needed tweaks though IMO. Driving is much better in this one than in GTA 3 and one thing I really love, but sometimes love to hate, is the inclusion of "tire damage". Your tires can get shot out/popped and when they do it can be very difficult, if not impossible, to control your vehicle. This makes those "lose the cops" missions sometimes very difficult.
The graphics in this game are top notch. They look very close to the GTA 3 graphics only better. I think it's pretty impressive what they've done with it on the PSP.
One small annoyance is "Loading..." of cut scenes especially when you've already watched them and are just re-trying a mission.
All in all the game is the best I've played for the PSP and I would recommend it to any PSP owner.