ArmA III is a game that improves on previous iterations in every aspect, the game looks better, is bigger, has more stuff in it, gives you great modding tools, and offers a unique experience.
All that does not mean that the game is for everyone, I play it because one of the first games I played was Operation Flashpoint it still is one of my favorite games, but that does not change the fact that both it and ArmA are not exactly well polished fun games. I do enjoy ArmA but not because it brings a smile to my face but because it gives you are sense of accomplishment, for overcoming the difficulties that the game throws you in.
Another interesting thing about the game is that you don't buy the game for what it is, but you buy it for what it will become mods are a thing that set this game apart from others they range from simple things like new vehicles and weapons, to total overhauls of the game like DayZ did with ArmA 2.
When you buy ArmA you are not buying a game you are buying a tool box, that you can use to create the experiences for you and your friends or enjoy the content created by others, very few games give you a chance to do that.
This game is a must buy for people who enjoyed any tittles created by Bohemia interactive, as for the people who have never tried a game made buy them I suggest looking up gameplay on Youtube or play the ArmA 2 demo available on steam before purchasing since the game is a world apart from other military shooters.