My best friend participated last year. She had already started getting her ideas down on paper and writing before November, but once NaNo hit, she used that to jump-start her unedited writing.

Be aware that it isn't only a stressful time for you as the writer, it takes a toll on your family and friends. Every time I called my friend to hang out, she said that she was at the bookstore writing (or at home writing or at the coffee shop writing...) and I could join her if I wanted to. I would go and watch her write and I would feel bad interrupting her. So, yes, not exactly my idea of quality friend time.

Be warned!