It'll give you a taste of what the game is like, however it limits the quality of the graphics and the fps you get...so it's nothing like the real game - but you get a feel for the gameplay. We're also playing Operation Arrowhead, which is a much-improved version of ArmA 2 (and a standalone expansion). Reading a review or watching a youtube video might give you a pretty good idea of what the game is about!
Make sure you have a good system. It's probably the most hardware demanding game on the market to date. A good system will let you play it at low settings. You really need an awesome PC to see the high and very high detail with playable FPS... But man it is a really beautiful game at full detail!