1 was great, 2 was much the same, brotherhood I haven't been bothered to finish, it's just dragging on and on and on....I'll do it one day buts its boring me.
I think the problem for me is, the first was pretty special, something we haven't seen before in a game as regards the whole concept, free running, graphics, scenarios etc.
To me it feels like they are running out of ideas to improve the actuall game and just filling what they already have with extra gimmicky gadgets to try and make it a bit different from the last one (flying machines, tanks, flame throwers, parachutes, etc etc...)