Well the Rainbow Six has long been categorized as 'tactical FPS', the same as SWAT (strating with the 3rd If I remember correctly?).
However, the gameplay, and game mechanics tend to differ.
One think that might join dimonkey's bad impressions, is that, as every Ubisoft, title, the game heavily focus itself on single player with a heavily scripted/guided playthrough.
The best you could do to convince yourself of the rather different overall feeling in the games of the series is to try some oh them: 'Vegas 2' for example (the only one I touched which provided me with a nice new game for a few weeks).
The spirit and the guts and feelings are somewhat different.
However, maybe am I wrong: please test it and provide the community with your feedback, and let us see what happens next...