ARMA: Combat Operations is the sequel to ARMA: Cold War Assault (aka Operation Flashpoint) and features quite a bit more realism than its predecessor, although the learning curve somehow manages to remain manageable. Anyone can play this game.
What's in store for you is quite possibly one of the most fun multiplayer modes in any game, ever. Although the GameSpy shutdown makes it a lot harder to host and join servers, if you can find someone willing to host a private game and get a group of friends together and play, you are in for an incredibly entertaining, yet challenging and tactical experience.
However, I found the singleplayer gameplay severely lacking compared to Cold War Assault. Unlike Cold War Assault, Combat Operations' campaign and single player missions are riddled with issues and AI problems, language and translation issues (especially in mission briefings), a much less engaging storyline, a generic soundtrack (some of which are bad remixes of Cold War Assault tracks), poor mission design/scripting, and many more flaws which make the singleplayer mode not really worth playing.
Combat Operations features a highly in-depth, yet also very easy to learn mission editor. The basic format of the mission editor has been pretty much identical across the ARMA series. You can create absolutely anything, and then with just a few clicks export the mission to singleplayer and multiplayer modes. I can say that me and my friends have spent far more time playing our own missions than the game's default multiplayer scenarios (and not because those are bad, either). The mission editor practically made this game for us.
Overall, I think ARMA: Combat Operations is an incredible achievement in terms of just how much entertainment value you can get from the multiplayer mode. However, the singleplayer mode feels bland, boring, and poorly designed. For 8 dollars, though, this game is an absolute steal because of how good the multiplayer gameplay is with friends.
Be warned, however. At least one of my friends has suffered from technical issues with this game that render it unplayable. This occurs most often on modern Windows 8 computers. Use caution if you are interested in buying this game.