I really enjoyed it. If you like comic book stories, you'll absolutely love this game and its expansion, The Third Reich (or something similar)
Even if you just want good gameplay, it's very enjoyable. Your heroes levels up, developing new abilities as you progress through the game. I heard Marvel Ultimate Alliance was said to be a ripoff of Freedom Force, but I never played that.
Edit: Oh, saw a reply earlier indicating that I'm thinking of the wrong game (Freedom Force, not Freedom Fighters). Well... shit