Best combat sim ever!!!
This is a completely free environment with a gaming area of 400square miles. With terrain ranging from desert to dense forest, there is plenty of variety for combat situations. It is reputed to take 45 mins of real-time to run across it. (from BIS WIKI website)
There are cities and some enter able buildings but most of the action takes place outside and so you can use about 40 different vehicles (boats, trucks, cars, bikes, planes, helicopters, APC's and of course tanks)
There are also a huge number of accurately depicted weapons with realistic ballistic dynamics including ricochets and bullet penetration through soft materials such as wood structures. The sound is also calculated to have Doppler effect and being hidden by objects.
This is NOT a shoot'em up. If you run into combat like Rambo the enemy will be laughing as they mow you down. The AI behaves quite realistically at times, taking cover while others use suppressive fire and they do try and outflank you. Sometimes they will stand about for a second before reacting to you but any delay in taking them out will result in your instant death as they are usually deadly accurate once they've targeted you.
It comes with a mission editor that allows you to create anything from quick battles to complex multi-objective missions with cut-scenes. Scripting is involved for complex stuff but much can be done just with the icon based editing.
ARMA really shines online in a good server.
One great moment I remember online was sitting on the outside seat of an AH-6 LittleBird helicopter flying low at dawn. I looked back and saw another LittleBird following as we dove and swerved along the canyons before being dropped off at the landing zone near a town.
CONCLUSION:
If you want total immersion in a combat sim then THIS IS IT...
It does need a powerful PC and GfxCard to run near full settings but it still looks great with medium settings. Some complain that the animation changes eg. from prone to running are a bit slow and you are very vulnerable while reloading but I think these are quite realistic.
There are some minor bugs but free updates are constantly being produced with great improvements. There is also a very strong modding community creating anything from missions and maps to new weapons and vehicles.