Don't buy it for the campaign. Its not good. The only reason you would buy it is to play online with people and to play custom missions. If you do intend on buying it get Combined Operations so you have Arma 2 and Operation Arrowhead. You'll also need a beastly PC to run it with a good frame rate.
Its a game built off of realism, not run and gun. It requires team work and patience to play it. If you're a Call of Duty run around and jump in the air whilst shooting people you will not enjoy it.
Personally I wouldn't buy it unless you plan on joining a realism unit or you have a group of friends that play it. Its not really a fun game to play alone or to just join random servers unless you have some buds with you on Ventrilo or TS3.
Also if you do get it you will need to download the ACE2 Mod. Most servers run it and it improves the game by like %400. More realistic weapon damage, weight, stamina, vehicle armor values, medical system, additional weapons and explosives, an improved backpack system. Tons of great stuff.