Don't be fooled by other reviews, this game is a must-have, and makes many improvements over the first!
Let's start with the latter. I can assure you, the series is far from ruined. Framerate issues are hardly noticeable - and I don't mean it like 'Mass Effect's framerate issues are hardly noticeable', I'm being serious - the framerate 'issue' is not really an issue.
As for the former, I have a theory. Sequels, of any genre, apparently have an additional facet that is judged when they are reviewed. It must be human instinct to compare a sequel to its predecessor, but I sometimes wonder why people are looking for something completely new and innovative in a game or movie that is a sequel. If a game goes too far off the reservation, people hate it; but, if it doesn't do enough new or 'take enough risks', people still hate it. It's an almost impossible balance to strike, particularly when the previous title was Rainbow Six: Vegas, the 2006 Shooter of the Year.
Personally, I think I think such a standard is a boat full of carp. Now, don't misunderstand: Expectations of a game are important, and I'm all in favor of comparing the game to its predecessor, but it shouldn't be any different than comparing it to any other game. Dinging a game on a review just because it's a sequel, and dressing it up as some cool cliche, like, "been there, done that," just doesn't make a whole lot of sense. I mean, it's called 'Rainbow Six: Vegas 2', not 'New Game X'.
Now that I've beat on that issue until it's good and dead, let's move on to the review.
The Good:
Vegas 2 does almost everything right. The campaign is a lot of fun. You'll find that the dynamics have been tweaked, which has a considerable impact on the game. Vegas 2 adds the concept of penetration, or soft cover, to the game. Enemies can shoot through light cover - this includes sheet-rocked corners and temporary partitions, among many other things. It really changes the concept of hanging out behind a sign or a corner and waiting to pick off your enemies.
Another addition is that your teammates now have a limited amount of gadgets to use between them. This may have an effect on higher difficulty levels, but outfitting stations are very plentiful in Vegas 2. They can also pop smoke, frags, and flashbangs in any place you tell them. Which brings us to the AI.
Gamespot hit the nail on the head when describing your partner AI as mostly helpful, but occasionally retarded (paraphrase). Your partner AI is very adept at acquiring targets. It also does a much better job of using cover than it did in Vegas. Your partner AI will now make use of cover along the way while executing move order across the room. The infiltration ROI also seems to be a bit more effective (e.g. your partners actually return fire) than it was it the original, but that just may be because my personal style of play has changed from one game to the next.
The enemy AI has become emboldened in Vegas 2. They're better shots, they track you better, they advance in a tactical fashion, and they will shoot you through soft cover. You really have to make use of your team and your environment and think on the fly to progress through the game.
The levels are fun and new, as well. Each one is laid out in a very believable way, and has plenty of cover to move through. The graphics are much more rich and smooth than the first. I know a few reviewers have left comments about the brightness, but the brightness is adjustable, and darkness is just a part of the game - use night vision or thermal vision.
One big change is how the PEC extends to the entire game. I won't get too deep into that, but it's really a lot of fun. The new experience system that allows you to unlock different types of weapons independently depending on the types of tactics you use, while gaining general experience to unlock gear, is a lot of fun, too - it's something that I wish they would have had on the original Vegas.
There's a few new multiplayer modes and maps, but I haven't spent too much time on them. The cooperative play is a lot of fun, too. It feels much more complete than the campaign co-op felt in the original Vegas. However, as the GameSpot review noted, only one player can give commands to the team, which will leave many players feeling like a third - er, fourth wheel.
The Bad:
Well, the worst part of Vegas 2 has to be how the enemies can spawn literally right before your eyes. It doesn't happen nearly as often as it did in the original. The framerate thing is forgivable, but it also has to be factored in.
Conclusion:
Vegas 2 is definitely a great game worth buying. It has a few flaws which you might find a bit obnoxious, but they're overshadowed by the great gameplay, story, graphics, co-op, mulitiplayer, experience system, and customization that Vegas 2 offers.
Honestly, I'd like to give Vegas 2 a 9.25, as I don't think it's quite a 9.0 or 9.5 (at least, not right now), but I'm going to buck the trend here and not ding on the game just because it's a sequel. There's easily 50+ hours of high-quality gameplay here - Enjoy!