I used to play Shell Shock Live heavily. I was addicted to it and I played it every day for several months. However, I was a bit bored and stop playing it six months ago or so.
I was a bit addicted to SSL too, but I never played alone. For a few months I'd play it everyday with some friends. But once they stopped playing it so did I. It's not as fun playing alone.
I was addicted to Exit Path for a few days... I was able to achieve Grandmaster status just by playing for hours on end every day for about four days! It really wasn't healthy, so I decided to quit for good after I became a Grandmaster.