ARMA2 is a realistic military simulator, probably the most realistic on the market. It's not just infantry combat, although you can play it that way. ARMA2 also has cars, trucks, tanks, helicopters, planes, and more. It even models ballistics and bullet drop, if someone fires at you from 200 meters away you even hear the bullet hit the ground before you hear the rifle. It's a big sandbox, which is good and bad.
The game ships with 2 islands, a large and a small. Both have forests, hills, roads, towns, fields, mountains, etc. I'd say you would spend a good 15 minutes driving from one side of the island to the other. If you can see it you can go there, there are NO invisible walls. The game locations are fictional, although they are based on real-life topography. ARMA2 is set in Eastern Europe, Operation Arrowhead is set in the Middle East. The possibilities on the huge playing areas are almost endless. You can also download user-created islands, some of which are impressive in both scope and detail.
A mission designer is included to make your own missions, along with sophisticated scripting capabilities. In addition, there are hundreds of user-created mods and missions, everything from a simple "destroy the enemy" mission to new weapons, vehicles, sounds, etc, to new game modes like "capture the island" and a GTA clone.
In a way there are almost too many possibilities between official and user-made content, and the quality varies. The official missions have some neat tricks, although they don't expose you to everything the game has to offer. Some things work well, others don't. You'll have to find a way to manage missions/mods and all your time gathering, installing, and playing all the different things. A launcher like "ARMA II Launcher" helps you manage what mods you want to use. There is plenty of help on the official Bohemia Interactive forums, although they are picky about posting in the appropriate forums.
Although the visuals and presentation are quite good, ARMA2 is quirky and not without bugs and oddities. Just mapping the controls is an adventure, what's the difference between "Prone" and "Go Prone?" You'll use practically every key on the keyboard, although remember you can map something like Control-G for landing gear or Control-E to eject. With a large playing area and so much going on, performance is also an issue, ARMA2 benefits from tweaking and downloading mods to do things like get rid of motion blur and tame the tall grass a bit.
There is plenty to do in this game for both single player and multiplayer. Setting up local, LAN, or Internet games is smooth and easy. It even remembers the last IP address you manually typed in (I'm looking at you OFP Dragon Rising!) Co-op missions, deathmatch, capture the flag, and more are all there, and with mission designers being able to do scripting plenty of different game modes are out there. There are "Warfare" or "capture the island" missions that add strategic and real-time-strategy elements as you build your base, build units and vehicles, and try to take over towns while searching for the enemy base. You can choose to be a soldier and take orders, command a squad, or be in high command mode and command all the units. The learning curve on Warfare modes is moderately high and documentation is scarce, but they are very unique and rewarding.
If you have the time and patience and like realistic military combat simulators, ARMA2 will delight and amaze you. If you're a run-and-gun sort of player with a short attention span, this probably isn't your game. It also helps to be somewhat computer knowledgeable and have good hardware. I have a 2.4 GHz dual core AMD, 4GB of RAM, Windows XP, and a GTX 250 video card. With tweaking it runs acceptably on my system, but I wouldn't want to play with much less. I'm saving my dollars for a solid state drive (SSD), from what I read ARMA2 does a lot of small file reads and greatly benefits from the almost 0 access time of SSDs.
