I have only played the game for about 10 hours but i can say that it is a really fun game!
To start off i will have to say the single player story is not as good as the first game. It is still fine though, just not as good. The campaign is also harder. Just peeking around cover for even a second can get you killed. The AI is for the most part smart but can make a few dumb moves. if i stick out my head for a split second a sniper could blow my head off but any other soldiers dont make a move. I think you are more dependable on your teammates in this one becuase they can get your butt out of trouble and i think their is a hidden agro meter like in army of 2 becuase they can divert the enemy away from me. Over all this is a pretty good single player experience.
The multiplayer i think is better than the first. The co-op terrorist hunt maps are bigger and better. There is more of a challenge and you can actually find good teamates unlike in the other rainbow six where people bum rush 6 guys and die. The co-op story is not as good but it is playable. I find it a little better than playing by myself though. Instead of 4 people playing together it is 2 and 2 AI. It makes it so you can experience cut scenes but dampans the fun a bit. I think what most people like to do is let you stick with them and send teammates somewhere else. The competitive multiplayer keeps most of the same modes but adds 2 new ones. Over all the multiplayer will keep me interested in this game for a long while.
I say buy this game if you liked the first or want to try and get into the series.