It's fun; probably second to Onslaught for me in terms of the DLC. Playing as the creatures isn't as bad as was feared, since you can break powerful enemies out of red crystals to help you out. The only real downside is that the agents get time reducing crystals to break (which shortens the game dramatically) and that the mode is not well-suited for all maps.
It's the best game mode in my opinon. It's pretty well balanced and getting a kill as the monsters is incredibly satisfying. The time reductions do make the game a lot shorter and I wish the host was given the ability to choose how many rounds there were per match.
siege mode is so worth. and it's not even hard to get a kill as a creature. i find myself playing mainly as agent, but constantly shuffling between a lot of the characters since many are very useful, depending on the stage.