ArmA II has many performance issues and bugs. Besides this, it is an awesome game.
ArmA II is a modern fps military simulator and spiritual successor to Operation Flashpoint 1. It is very detailed and for hardcore tactical gamers only. This game requires one to tweak it and install lots of mods (with latest patches), to overcome the bugs and performance issues which is evident in ArmA II.
Pros:
Easy to use and very flexible Editor
Addictive
Good Multiplayer
Very detailed
Some nice graphics
Loads of vehicles, features and weapons
Challenging
Cons:
Buggy animations, sometimes
BUGS!!!!
A massive system hog. It requires a lot of mods, tweaks and patches to get rid of the bugs and make this game smooth.
Some bad graphical features
Not very cinematic in gameplay (lacks some presentation)
Not everyone will like ArmA II, but for those who like tactical fps games, then you'll love ArmA II.