As I said in my review of Rainbow Six: Lockdown, I buy every one of these games, despite not being a fan. The most recent installment is a big improvement, though still rife with flaws and frustrations.
The major improvement this game brings to the series is the ability to use cover. By holding the right mouse button you can lean against a wall, crate, etc and stick to it, using it as cover, going into a third person view, and leaning out or going over the top to fire. It's a very intuitive system, and it gives a feel of tactical movement as you sweep through buildings. The enemies use the same system, and it adds to the realism of the game overall. A very nice system.
The squad AI is also better than in the past, though nothing special. The bugs of Lockdown are gone, no more AI accidentally fragging itself, wandering off, etc. Generally they will use cover pretty well, follow your orders, and effectively clear rooms. The door stacking commands are intuitive and easy, though at times there can be issues with overloading of keys. For example, the Q key exits the “snake cam” mode for looking under doors, and it also throws grenades. That leads to exiting snake cam, and promptly tossing a grenade into a closed door. The Spacebar issues a stack up command for a door, but it also opens it if you're closer, so at times you'll go to trigger a two pronged attack from two seperate doors, and as you attempt to open your door, you issue a stack up order and the team comes over to your door. All in all, though, they do what you tell them, and if they get killed, it's because you told them to do something bad.
You and your teammates also are much hardier than previous versions of the game. Now, if you're hit, your vision goes blurry for a few seconds, and if you get into cover, you will eventually “heal” up. Similarly, if your teammates are badly injured, they will lay there waiting for you, or the other teammate, to give them a magical injection which brings them back to life. It's nice that they're so easy to bring back, it makes you more willing to use them, but it also leads to some rather ridiculous goings on. Since they (and you) get shot so much in combat, gunfights end up sounding like strange violent porno films for all the grunting and moaning.
All that aside, there are still plenty of situations where the tactical engine really shines. You'll snake cam under a door, designate a terrorist for your mates to kill, launch a simultaneous entry from two different doors, and in a few seconds, the room will be clear without a return shot fired.
Speaking of engines, the graphics engine is a bit of a mystery. It's as if they deliberately ran some sort of “fuck the colors up” filter on it. While capable of looking very at points, the game spends most of its time being all muddy, washed out, dark, or otherwise hard to look at. Enemies can be very hard to see, and while this may be realistic, it seems exaggerated at times. I'd be tempted to think it was my monitor or video cards it's so bad, but since so many other games run and look fine, it has to be inentional.
While the colors are pretty poor, the animation and modelling are pretty top notch. There's a huge variety of moves that the main character and his teammates can do, leaning in and out of cover, breaching a door, rapelling, etc. etc. The reload animations for the guns are custumized to the weapon type, so you'll see your mate flip open the top of his M249 to load in a new belt... The main character can be seen speaking his commands when you issue them while in a third person view, and in that view you can see him looking around to mirror your mouse movements. Lots of nice touches that make jumping in and out of cover seem more realistic and dynamic.
Towards the end of the game, things get significantly more difficult, and much more dependant on poorly playtested set pieces. One, in particular, involves hacking a computer to prevent the villain's escape, and is keyed off killing all the terrorists in a given area. The problem is that you may kill a bunch, and one may just cower somewhere out of sight, and not show himself. Your teammate can sit there forever trying to hack the computer, but it's not about how fast he hacks, it's about killing all the terrorists to cue the next scripted event. You can't go looking for him, because you'll fail the mission. And the mission failure message isn't at all helpful. You have to keep redoing the mission until, somehow, it just works. Frustrating and highly disruptive to immersion.
A score of 6 means I was glad I played the game, but not by much. While this game is significantly better than Lockdown in terms of gameplay, it's really a step back graphically (or perhaps sideways thanks to the great animations), it suffers from some issues with the level design late in the game, the plot is pretty much worthless, and it's almost a year later, and thus a year harder to impress.