APB is a fun game to play, combining elements of GTA-style gameplay and MMO's, just don't play the game alone.
You start the game by picking a side: the enforcers or the criminals. As a criminal, your job is to cause trouble and disrupt enforcer jobs. As an enforcer, your job is to keep the city safe and thwart criminal efforts. One of the key problems here is that there is no real end game. There's no story here. There's no real arc or anything that leads you to do these things. You're simply doing it cause it's fun. That is a double edged sword. Yes, the game is a blast, but it can also become fairly repetitive. Leading to my next point: don't play alone.
As a solo game, it's rather boring, and even a bit tedious. You simply do missions, "level" up your character, and buy stuff. At times, it can be frustrating as well, when an APB goes out and a gang of criminals comes to thwart your mission. Suffice to say, without any backup, taking on a whole gang is no fun. However, that does bring up one of the cooler aspects of the game, and probably the line for people. Your battles fought here are not against computer opponents, but against players on the criminal side. While on a mission, you can almost always count on an APB to go out and to end up in a battle against other players. It's almost certain. But if you're looking for an MMO with PvP that is different from most others, this game should prove to be very fun.
Thankfully, the game allows you the use of vehicles, and multiple people can get in one vehicle, with passengers acting as gunners. It adds some interesting elements to the game and makes for some good fun. Another nifty little addition is the ability to add your own music to the game. The customization in the game is probably the biggest, an possibly best, point here. You have a huge amount of customization in the game, from customizable clothing, to custom vehicles, to your own character. You hardly ever feel like a clone here, and that goes a long way with becoming an individual in the game world.
APB is definately a lot of fun, but you really gotta play with a group of people. Otherwise, it can become boring and even frustrating. However, this game moves in a good direction proving that MMO's were meant to be played with other people.