Arma 2's engine is a bit weird, and the interface is unintuitive in the beginning. But the game play underneath is just second to none. This isn't some fast FPS game, its primary greatness is in open world infantry combat at 100's of men in coop mode. No other FPS comes close to offering such an authentic infantry simulation. The AI is smarter than any other game I have seen although that doesn't mean I consider it good.
I think the game is only really fun if you join with a group who has been making missions for years, knows how to put together and organise a group and plays tactically together. If you find that (tier1ops.eu is the one I am in) then you'll likely never go back to BF3/COD like games. Arma 2 comes from infantry simulation software and its pretty deep. You need to learn how to do things, like aim and fire.
For example to aim a sniper rifle takes hours of training and practice, it works similar to real life. That is you have to estimate wind, range and height difference and then use a ballistics table and adjust the guns scope to compensate for all the factors in order to hit your target. Placing a mine is equally complex involving knowing which tabs to pull to make it magnetic or IR detected and then pulling the ping and aiming it precisely. The scopes are pretty realistic as well including holo sights and RCO's that work the same way as they do in real life. I am always horrified seeing an ACOG in COD/BF3 that does not work correctly its down right annoying.
Personally I have been playing it since it came out. I think its the best FPS ever made, not because the animations or graphics are best, but because after all this time I still enjoy playing it because the mission scope is so wide. I get to run fireteams and squads and learn to use and abuse groups of equally trained individuals. Its a blast playing with the right people. I would recommend Tier 1 in Europe and Shacktac in the US.