Interesting story, multiplayer lacking.
Now going back to the single player. It is fun to play alone and with a friend. The co-op options allow for you to easily play it with a friend and just adds a sense of fun to the game that was missing in the first. The storyline leads you around Vegas with your squad around the same timeline as Vegas one, and does a good job of it. It's very interesting, but it doesn't reveal enough for you to fully understand it.
Now onto the rank system, which I think is TERRIBLE. It's way too easy, most people cheat with a single player glitch which allows you to get to Elite in a matter or hours. The ACES system is also garbage, allowing you to unlock the guns way too easily, most done also with glitches in the single player. In the first Vegas, it was cool when you saw an Elite. There were a lot, but you didn't see them everywhere. In Vegas 2, EVERYONE is an Elite, and there's nothing special about being one except the custom camo.
Then to end this all is the online play which is a step back from Vegas 1. It has no match making, which is good and bad at the same time. The good is easy to get with friends, ability to chose map, and just sit back and have fun. It's back because teams end up stacked, angry hosts kick anyone better than them, and all items end up restricted that the host can't counter. Above that, they took all customability out of the modes, bringing this game to a new low and ruins the experience.
My final verdict is rent it or get Vegas 1. Vegas 1 is better on so many levels, except for single player. In online modes, Vegas 2 does not compete and is a waste of money. Rent it for the single player, then buy Rainbow Six Vegas 1 for the multi player.