I'm participating again this year in National Novel Writing Month (I refuse to shorten it. This is my thing and I don't intend to force it on anyone else, but I don't like acronyms or neologisms.) I'm ahead of schedule at a little less than 5,000 words and working to build up a buffer.
There are people who are opposed to National Novel Writing Month on very reasonable grounds: that it encourages sloppy writing, that it facilitates people slacking on writing all year long and then trying to make up for it in one burst, that most of what comes out of it is utter drivel.
I think that's all more or less true, but my experience with it has been incredibly positive. I write faster with the deadlines, and before I began national novel writing month last year, I was paralyzingly afraid of diving into a long-form work. I'd written dozens of short stories- more than the word count necessary for a novel and a half- but I was completely unconvinced I could sustain characters or ideas over the time it took to tell a novel length story, not to mention working out the logistics of the more complicated plot and interaction a novel required.
Even though my first try had serious problems, I found myself really enjoying the kinds of subtle character growth you can't get outside of long form. The format of the challenge, fast, furious, and messy, encouraged me to get past my anxiety about not being good enough- one I had more or less gotten over in the format I was comfortable with.
I think if one wants to be good at any pursuit, it's important to push the limits of what you think you can do, to stretch outside the safe zones where you're comfortable. I think National Novel Writing Month, whether the novel results are ultimately worthwhile or not, pushes writers to grow, and as such, it's got my thumbs up.