One of the best comic book based games ever made. It's right up there with Hulk:Ultimate Destruction, The Darkness & Marvel vs. Capcom. I loved that it was actually based on comic lore and not one of the many movie/cartoon adaptions.
The freeflow combat system has to be one of the best combat systems ever developed and the stealth gameplay is excellent as well. Loved the Riddler's hidden trophy hunts and riddles as well as the "Spirit Of Arkham" side quest. Graphics are top-notch, some of the most detailed models you will ever see in a video game. The game's voice acting is excellent as well, but wouldn't expect any less when you've got Mark Hamil, Kevin Conroy and Arleen Sorkin.