Good, but don't call it a GTA killer.
Being on foot is much more fun than being in a car. There aren't many jumps or things to do stunts off of in the city, and the few jumps that are best to hit aren't very thrilling. However, when on foot, you can jump across rooftops, climb skyscrapers, or kick gang vehicles into the ocean. The only thing to do in vehicles is do the races, which aren't fun.
Shooting is simplified with the use of precision aiming. There are a few different types of guns to use, and you make them available by taking them from gangs to your supply points. Multiplayer lets you play with another agent by your side, or lets you do the work while the other player goes around climbing buildings. Nothing special in multiplayer but not terrible.
Unlocking achievements in the game seems to be the most fun thing to do, as they have you doing a wide variety of crazy activities like juggling cars with rocket launchers or impounding vehicles or killing people with building structures.
There is some decent fun to be had here, but not enough to warrant a large replay value.