First off I would like to extend my deepest apologies to the previous reviewer. This is likely your first pc game, you seem to be coming from the COD console crowd and are not aware that game in fact do have to be updated some of which are quite sizeable.
Arma 2:Combined Operations (C.O.) is a robust military simulator that despite it's age has massive community driven content and support behind it. Everything from the realism based mods like ACRE to the popular DayZ are examples of continued support for this title. Add to this the massive list of custom missions you can play it adds a huge aspect to the replay value of the title.
While it features a fairly steep learning curve and is a departure from the scaled back FPS genre if you are looking for a title that is akin to the classic Rainbow Six titles and favor planning and realism over run and gun shooters then this is the title you have been looking for.
As a previous reviewer pointed out there is in fact updates still being actively released by the developer despite it's 2009 release date. So you can expect to see the games features and issues fixed on an ongoing basis even as the company is in development of the games replacement Arma 3.
While the single player missions are essentially bot driven multiplayer maps the AI can make every mission a bit of the unexpected. Couple this with the huge battles available over multiplayer and you have a top notch online milsim that really shines once you get past the familiarization process.