Pros: Very innovative, unrivaled realism/authenticity, more content than any shooter I know of, mod support
Cons: Horrible AI, campaign and coop scenarios aren't anything great
So just how different is ArmA from a normal military shooter? To be honest, it's different in almost every way. It's fundamentally different; the goals of the game are different, the mechanics are different, it plays out differently. It's a simulator, the mechanics are far more complex than normal shooters. It uses almost every key on your keyboard, the pace is much more realistic. There are much more variables when playing, such as your endurance which affects aim sway, not to mention the ballistic system is far more realistic than pretty much any other shooter.
Other military shooters offer either a run-and-gun PvP experience (Battlefield, Call of Duty), or are focused on the single player campaign (Call of Duty 2 and earlier). ArmA 2 is neither of those. Although it has long campaigns (ArmA 2's took me about 40 hours, not sure about OA), they don't put too much effort in making great campaigns or great coop missions. Aside from the gameplay mechanics, the emphasis is placed on the mission editor.
This revolutionary editor dates back to OFP and as far as I understand, hasn't been changed much since then. It's very innovative in its design, and how it allows players to make missions. So get to learn it, because that's what the game is all about. The game emphasizes custom missions played strategically, in private coop servers. This is yet another big difference between ArmA and other shooters: it's not PvP, you don't want to be alone either, and public MP is not recommended. ArmA 2 emphasizes private coop on custom missions. Play with people you know are serious gamers.
Most modern military shooters, like Battlefield and CoD, are designed to be casual, mindless fun. ArmA 2 on the other hand is slower paced, missions can take hours, and it's very tactical and strategic. Because of this combined with the realism, the intensity it brings on is on a whole different level. No shooter comes close to this. The realism and authenticity forces you to play this game in a completely different way, compared to other shooters. You actually sprint for cover, you will be afraid of getting shot because one shot is all it takes to put you away, and there are no respawns. This intensity is awesome, especially in a private coop server. So there's no mindless fun to be found here, only intensity and the highly rewarding feeling of winning a long, strategic battle.
Sadly most people don't recommend playing this game, and not because ArmA 3 is on the horizon, but because of the Day Z mod. ArmA 2: CO has become a top selling PC title, but most of them don't play the game itself, they just play Day Z, which is nothing but multiplayer zombie deathmatch. Definitely not recommended.
If you get this game, be sure to get Play WithSIX (mod installer/updater), ACE 2, and Zeus AI for ACE. ACE 2 is a total conversion mod that enhances the game in many, many ways, such as new sound effects, lots of new guns, more realism, many new gameplay features, and much more. Zeus AI improves the AI by a LOT, but it still leaves much to be desired. Stealth gameplay is just impossible in ArmA 2. Also, you need a very powerful PC for this game. It's by far the most resource intensive game out there.
At this point, it isn't a bad idea to wait for ArmA 3. I seriously doubt it will be a proper sequel, but it will no doubt have a number of improvements.