I thought and all of the AC games are quite fun. The story is compelling and very interesting. Do I enjoy all aspects of all the games? Certainly no, but for the most part they have put together a great series, and added new bits to keep it relatively fresh. Didn't like the den/tower defense? All you had to do was keep your awareness at a minimum. Really wasn't that difficult to do. And once you got your den master to master assassin that den would never be attacked.
If you don't enjoy the series, then don't buy it. For me, I'm not a huge fan of the multiplayer, but it keeps me interested for a bit. I had some fun with it, but now I'm about tapped out and am trying to finish up that last few achievements so I can move onto another game.