I had no non-COD FPS experience before I played this game, so it was quite a learning curve. Unless you're used to ARMA 2 or a game like that, prepare to die, and die, and die some more. Be pleased if you don't finish a game at the very bottom of a server. But it is worth it.
It sounds great, it feels great, and the fact that you have a lot of Russian players shouting at you over the voice chat somehow enhances the realism!
BUT: Since the release of RO2, the number of people on RO2 servers has fallen dramatically, at peak time there will be a full Panzer server (where the players are impossible to beat imho!) and maybe a couple of infantry servers, but this'll continue to fall. The game offers a single player vs AI training mode, which is useful to learn about the maps but the bots themselves are appallingly useless.
The realism of this game is great though, only in ARMA 2 have I been more afraid of dying! It is very exciting to play, and can involve some great teamwork:
You spawn, you run to the nearest half-track, joining your comrades in the back you speed through the fields towards the front. Artillery rains down around you. An enemy tank is seen in the distance and fires. Bailing out the smoking vehicle, you dive into the hedge. Crawling along this, you meet a support trench that takes you closer to the action. Bullets zip above your head. Where are they coming from? You wriggle up to a fellow machine-gunner, and resupply him with ammunition. Whilst he changes the smoking barrel, you fire off a few rounds with your semi-automatic rifle to keep their heads down. More friendly riflemen join you. Fixing your bayonet, you leap out of the trench and sprint towards the nearest building. A bullet catches you in your hand, causing you to drop your rifle, leaving you defenceless. You pause momentarily, and an enemy sniper hits you. Cue shot of your body falling to the ground. Repeat x many.
But was it fun? YES