These Batman games are some of the best super hero games ever made. Great stories.
However, hindsight being what it is, my biggest complaint would be that the hide and seek clues, riddles, question marks, etc. make up WAY too much of the game play. Remove those elements from the games, and I think you have 4-6 hours of storyline per game.
It looks like two more games are on the way. A 3rd game in the Arkham story line and a 50-60's Batman made by the guys who did the first two Arkham games.