Fun game to play with a friend.
Although the first couple missions do not allow you to select which hero you will use, by the end of the game you'll get the chance to play as several different JLA characters. Don't expect to play the entire game as your favourite Justice Leaguer; this game is all about teamwork. At first I was a bit dissapointed that I could not choose to play Batman for every mission, but soon began to enjoy the variety of superpowers used by Zantanna, the Martian Manhunter, Hawkgirl, and others.
While the single player mode is enjoyable, it's the two player mode of this game that really shines. Each of you takes control of one character, and you can switch at any time. Experience can be divided equally between the two characters, or at any fraction that you choose (for example, you can set it such that a weaker player receives 70% of the XP from an enemy, while the stronger player gets 30%). Have fun with this game. Don't expect anything revolutionary or even particularly original. But seriously, it doesn't matter. It's fun. Enjoy.