Arma 2 is a really cool game, especially if you play some multiplayer servers that use custom mods/scripts. (Some of the RPG servers come to mind)
Really, no modern shooter can compare in terms of realism. Sure the graphics aren't Call of Duty, but the sheer scale of it all and attention to on-foot, wheeled and tracked vehicle, and aircraft simulation is simply wonderful.
For people new to the series I would recommend Operation Arrowhead. They enhanced the graphics in it and fixed some other bugs, I also think the single player campaign is tightened up more.
My only complaint, and it's a huge one, is that female models can't run or hold weapons. I understand Bohemia Interactive not having female ground troops, however they have LOADS of support units and convoys in which women soldiers DO operate in real life, not to mention the insurgent troops which should have no sex preference. Also there are plenty of female pilots in real life too, of course BIS is oblivious to this. BIS is just clearly sexist, really.