Rainbow Six is far more a tactical game than your standard FPS. The emphasis is on careful planning before the mission rather, and if you simple charge in guns blazing, either you, or a lot of hostages are going to die. It's much more a game based on intelligence, stealth and timing.
Each mission presents you with a complete layout of a given area. You select soldiers based on their skill-sets, select from a wide array of weapons and then plan your egress. The map takes a bit of getting used to, but once you get the hang of it, you can plan a complicated series of actions that will have your team liquidating terrorists, flashbanging rooms and disarming bombs. The AI of your soldiers is sometimes a bit iffy. While they will often spot targets and take them out, often saving your life in the process, they tend, on occasion to get stuck in doors, but you do have a wide array of commands you can give them that they tend to execute well. The enemies, like-wise, have some AI issues. They have nigh perfect aim from any distance, so you'll find yourself trying again and again on a certain areas, but will sometimes just stand ramrod stiff while you and your team wipe them out.
For those whose experience with the FPS genre begins and ends with games like HALO and Duke Nukem, you'll be in for a very steep learning curve. But once you get it down, Rainbow Six is a tense gameplay experience with a rewarding plot and lots of fun and realistic scenarios.