I haven't played it myself, but I've been reading various impressions from others who have the game already. The main complaint is that it's very linear and almost everything is context sensitive. You can't actually use your grapple or batarang whenever you want to, only at select times designated in the game.
Also, the game revolves around the fear aspect. You have to trigger certain events during the level to scare thugs into dropping their weapons. It's apparently suicide to take on enemies before doing so, as they will kick your ass quite easily (Batman dies in 3-4 shots from a gun.)
Those that have beaten it have suggested it will take most people 8-10 hours to complete on the normal difficulty. Unlockables include 3 different costumes and some interviews with the movie cast.
If you're a Batman fan, it's probably a given you'll pick it up - everyone is saying it's the best Batman game ever (which isn't saying much.) However, most are still disappointed at some aspects of the game and suggest a rental for those non-fans.
Edit: The PS2 version comes with a ticket to see the movie for a limited time.
