Not quite everybody's cup of tea.
With that said, let's begin on the subject of realism. The crack of an AK, and dull throb of a far away chopper, and the sounds of a SAW ripping through it's ammo belt are all there. The firefights only last a few seconds if somebody has their act together, the optics and iron-sights behave like they do in real life, and the player's movement is dead-nuts real. Ballistics are half there; Chernarus lacks wind. Bullet-drop, however, is alive and present. Unfortunately, Bohemia Interactive chose not to integrate scope-adjustments, so a sniper has to rely on the mil-dot system.
ArmA 2 is extremely well designed. Elements of RTS and RPG are present, while you can choose to play it any way you like, the most common being from a third- or first-person view. Bohemia Interactive was wise enough to include an editor in the game, so replayability isn't a question.
The multiplayer is where it starts to get really interesting. Most games boil down into two types; whether your teammates are tactically competent, or whether they're not. Obviously, a person and an A.I are two different animals. With a human player, you can leapfrog, slink around, and show a clear LZ with green smoke. With an A.I, not so much.
The A.I is, in my belief, the only area of this game that is liking. ArmA 2 boasts a no-scripted-path approach to the game, and they're right. That also proves to be a problem, occasionally. Units have difficulty moving around obstacles, and once they're stuck, they're out of the fight. Ordering your driver to make a left turn proves to be a hassle more often than not, so that eventually it breaks down to you juggling positions inside of an Abrams.
Overall, if you're a hard-core, balls-of-steel kind of operator who's fascinated with doing this stuff for real, or seeing what it's like, purchase ArmA 2.