One of the biggest complaints with the original Vegas was the lack of a seamless cooperative experience. While the single player campaign was available in 4-player co-op, you could only do missions individually and then drop back to the menu. Rainbow Six Vegas 2 corrects this by making it a seamless experience, but only allows two players to play through the entire game. The 4-player co-op is still available in the terrorist hunt missions, which pit your team against a set number of terrorists on a multiplayer map.
For those that have never played a Rainbow Six game, the series is a tactical shooter with tons of guns and some incredibly realistic damage and weapon modelling. The first Vegas introduced one of the best implementations of a cover system in a shooter to date, simply hold the left trigger to hug cover, while the left analog stick moves you out of cover in the direction you press. Working with teammates is critical as timing is everything with entering areas. You need to make sure all directions are covered, and you'll need to make sure the tangos are dropped quickly. This game naturally enforces cooperative aspects. In fact, there were times when I found myself working with a teammate doing things that Army of Two built in as a feature; for instance the co-op sniping aspect. Instead of a button to engage it, I'd simply count down and call out targets.
One thing I found odd is only the primary player is capable of controlling the AI teammates. And while you can revive your AI teammates, you can't revive your co-op partner, instead he has a 10 second re spawn timer. If both players die within this time period it's game over. It's a minor annoyance, but it just seems kind of silly that the developers put all this effort to make the co-op integrated into the story - and then remove the player from the story with a cheap tactic like that.
Fans of tactical shooters and action games will enjoy pairing up with their buddy as Bishop (player 1) and Knight (player 2) through the streets of Vegas. Despite a few minor annoyances the cooperative mode is tight and refined, and the ability to import your face to your character makes things even more engaging.
What happens in Vegas stays in Vegas? From the sound of things that will be Rainbow Six Vegas 3.
There's not too much to add to what Nick says above. The most disappointing part of the game for me was the re spawn mechanic mentioned above. It makes the game much easier and reduces the penalty for dying to nearly nothing. A nice thing about being required to revive your teammate is that it also penalizes the player that hasn't died. So you get not only the benefits of having an extra player, but there are extra challenges, giving you more motivation to work as a team and to be around to support your teammate.
Still, there are plenty of opportunities to work together. Nearly every room you enter has at least two ways in. I found that what we did the most however was to send the computer-controlled players to one door while Nick and I stormed the other. The are also a few scenarios where it really pays to split up and these are handled well, however if you are killed you re spawn right next to your teammate regardless of where you were before.
The levelling of your character and the customization aspects adds a fun RPG element to the game. Killing enemies in different ways earns you different types of experience points, which keeps you switching up tactics (the messages that pop up when you kill enemies serve as a kind of psychic notification that an enemy has died, especially for things like "Kill through cover" or "Kill with explosives"). The gameplay itself becomes repetitive, but the variations in level look and feel do a decent job of keeping things interesting.
Overall, I think Rainbow Six Vegas 2 is an excellent single player game that does an admirable job of including a co-op mode. Drop-in/drop-out co-op makes it easy for anyone to try out playing through with a partner. It would have been nice if the co-op experience added something new to the single player story, or if a bit more attention was focused on the co-op mode (for xbox 360, there are a total of 3 achievements that net you 35 points for co-op, while the majority of the game's 47 achievements are awarded for getting kills in deathmatch games). Still, the included co-op is executed well, and got me into a game which otherwise wouldn't have been as interesting to me.