RS:V2 is not that much different than the first, but its still worth a shot for fans or people who missed the first.
First off, despite the "2" in the title, Rainbow Six: Vegas 2 is no sequel it's a prequel. This is somewhat confusing as to why they did this, but the game does play right in the first really well. You start the game off five years prior to the game in france where you Bishop are in charge of two recruits one who is Logan Keller the team leader from the first game and the other turns out to be the main antagonist of the game. As far as story line goes its pretty much just Terrorists… weapons… in Las Vegas, that's pretty much all you really need to know. Despite being named after Tom Clancy, the story isn't all too engaging.
Game play wise, the gunplay is a lot slower than other modern FPS like Call of Duty 4. This isn't a bad thing by any means. RS:V2 is more about tactics rather than run and gunning. Instead you'll be stopping at every door you come to looking through with a camera, telling your teammates what to do, and carrying out your tactics. Ubisoft has described Rainbow Six as "the thinking man's shooter" which is possibly the best way to describe the game.
Along with the slower pace, you'll find the game a bit harder than other FPS. If you screw up your tactics and burst in a room with out a plan, you might be a little screwed. But when you do kill someone with your tactics you'll be rewarded with experience much like you are every time you kill someone in CoD4 online, except this time you can get exp online and offline. With higher rank you unlock better weapons and better equipment that like your experience you can use these items online and off.
Online has a ton of stuff to do. There are lots of maps and lots of modes. There are your standard team death matches, set off/defuse a bomb, ect. Also there is a mode where you and some others must go through out the map getting rid of terrorists that are place around the map. It will take you a while to get bored with online being that there's just so much to do. If you're looking for something to take you away from CoD4 then this may be it.
On the negative side, I've experienced a few problems quite early in the game. First off in the strip club level I experienced a high level of frame rate issues a long with sounds cutting off and not even play (like enemy gunfire.) And from what I've heard from nearly everyone, the game isn't exactly all that newer than the first one.
Overall, RS:V2 is one great shooter that just lacks overall innovation to separate it from the last one. If you were a fan of the last one, or just didn't experience the last one, you will probably enjoy this one as well.