Advertising in general has little effect on whether or not I will play a game. Other factors, such as word of mouth and reviews by people with similar tastes to carry far more weight than advertisements. The large, lavish ad campaigns you see on TV and mainstream web sites, such as news sites, etc. aren't targeted at existing fans of MMO's; they are targeted at those who aren't fans. In effect, these ads serve to "mainstream" MMO's.

That said, "Flash ads offering 7/10/14 day trials", will often get me to look at games that I've passed over in the past. It's easier to talk a friend or two to try out a new game with me as well. Many, including myself, don't want to have to buy a game just to see if it is appealing.