So far, I'm very impressed with Stack Overflow. It's a great source of information (many of my more obscure development-related Google queries wind up there), and a good way to promote yourself while helping build the already useful repository of information and advice.

My original plan was to post regularly on Rails Forum, but to be honest Stack Overflow appeals more. There's a broader range of technologies represented there, and the reputation system gives me a direct incentive to continue answering questions. So, I've added a little Stack Overflow block to the right-hand column of my blog, & I'll see how my time & motivation goes w.r.t. growing my reputation.