Well, I finished Chaos Theory and now looking forward to playing through Double Agent.
CT was good, it definitely builds up on everything in the original and improves on it. I did miss the ability to shoot from behind corners/walls (though maybe I just missed something). One thing that sorta bugged me about this one, just as it did in the original, maybe it's just my style of gameplay, but I rarely use the "cool" moves. I never used the wall split in this game, and only once in the original, and I only used the "pull from off the ledge" and "choke from overhead pipe" moves twice, each. Still overall, great atmosphere and cool gameplay.
If you think Assassin's Creed is revolutionary you should play these games, AC definitely does some original stuff, but it also borrows tons from SC.