This is definitely the best game in its series (from OFP to ARMA).
ARMA2 is great game for long cooperative weekends where you and your friends play through some different scenarios. However it is very hard to find people who enjoy long and uneventful parts of this game. It's definitely bad choice for casual gamers. This is even why I dropped this game for Bad Company 2. At some point game must be a game. ARMA2 doesn't know what it is. It seems like a "dumbed-down" application for military training (for tacticians I guess) that just doesn't work well as a "game", but still is a great piece of software for whoever wants realistic experience (real-time) of possible on-foot military scenarios. I enjoyed Operation Flashpoint Dragon Rising singleplayer experience more then in ARMA2 which also claims to be milsim. So better rent OFDR then ARMA2 if you are not willing to try long and cumbersome multiplayer sessions of ARMA2.