I dunno man, how long it can be fun? They can make only so much animations.
To me, personally, the franchise was dead halfway through Relevations. I finished it just because I wanted to know how Ezio's story ends, I was having no fun with the gameplay itself. The parkour with its auto jumping, auto grabbing and auto everything was boring, all I had to to do (PC) was hold down Shift+Space+RMB at all times and no obstacle would pose any kind of danger, Ezio just runs over everything on himself. How do I not fall asleep doing that, the game was practically playing itself.
Combat was even worse, as mentioned by Supernova above. One shoot one kill counters ruined the combat with the enemies politely attacking you one by one, they were slow as fook too. Jesus christ. Also, at this point the franchise had barely any stealth left in it.
Now, I'm not hating, AC2 is actually one of my favorite games ever, but it just got so stale with each and every game copying AC2 with a minor tweak like a hookblade or whatever, that I just couldn't bear it, let alone pay for this Call of Duty situation.