I finished the Starcraft 2 campaign and I've started playing the multiplayer now. I'll keep going as long as it's fun. My guess is it'll stop being fun around the same time that I need to memorize build orders and learning the crazy SC-lingo I've been seeing on twitter. At that point I'll probably just go back to playing against equally incompetent real-life friends instead of braving the battle.net waters. 
 
I wonder how many people are actually gonna try and get GOOD at Starcraft 2. Personally, with the kind of games that require signing away your soul to reach a certain level of proficiency (Street Fighter is another one of these), I just end up plateauing fairly early.