There's a whole writing community that I never would have found if I hadn't done NaNo, and my writing would be the poorer for it. NaNoWriMo is helping me get over my intense fear of sharing my writing. I used to be exceptionally paranoid even among other writers I knew when it came to sharing my writing. This is helping.
It hasn't been entirely positive, and the forums can be an extremely distracting place. However, the "Word Wars, Prompts, and Sprints" and the "Adoptables" forums have been extremely, AMAZINGLY helpful in getting new ideas, forcing myself to write, and beating my own personal procrastination monster.
Every year around this time, I see a flurry of posts on NaNoWriMo -- why you should do it, why you shouldn't, etc. Most people do the plug about how it forces you to speed-write 50k in a month, which helps you focus on discipline and ignore perfectionism. It does do those things -- but I've never seen or heard anyone to recommend it for the writing community.
So I will! :) The best part of NaNoWriMo is that all the other people on the forums can be extremely useful, helpful, and fun. Of course you have your nasty people, too, but the moderators generally do a good job. (I can recall only one time I was really and genuinely offended by a moderator's response to my concern.) And the best thing about the forums is that they don't close when November ends. There's a small but dedicated group of people who hang out on the forums all year long.
Have you done NaNo? Have you "won" NaNo? Did you use the forums at all?