I've used Stack Overflow for quite some time now. It's been a great resource for me (and probably most developers). Up until recently, however, I had used it mainly through Google -- I'd often come across Stack Overflow results when searching for how I might solve a problem. I'd also ask the occasional question, too.
A couple of weeks ago, though, I started to realize that I could answer a lot of the questions asked about iPhone/iPad development. I had run across a lot of the same questions that are asked on Stack Overflow and was able to answer questions with decent code snippets. As I started to do so, my reputation points started increasing and... it totally became a game for me. I had heard of developers frequenting the site daily (even hourly!), looking for questions to answer or thinking of good questions to ask. I never really understood that. But now, having given it a chance, I can see where the addiction comes from. When there's a lull in my coding or reading the various news sites I read, I find myself skimming through questions on Stack Overflow and answering questions.