Here, took some screenshots with AA set OFF, LOW and HIGH. Between off and low, absolutely NO difference but high you can see the difference. This means that it is implemented but maybe not the best. Overall though, set to high I haven't had any performance hit but of course your results may vary based upon a variety of factors like but limited to hardware, drivers, background software and unknown software/hardware bugs.
Comparative GIF
Had to use an alternative since Imgur is blacked out due to sopa/pipa protest.