Ah, NaNoWriMo. We had the best of times, we had the worst of times.I don't know if I'll do it again this year, but I found it immensely useful when trying it for the first time last year. For years I had allowed life's distractions to keep me from achieving anything remotely novel-length, being quite satisfied with short stories.
As artificial and arbitrary as it was, the notion of a deadline, and of setting a hard wordcount goal, made all the difference. A graph on the fridge that I updated each night with my progress, the act of posting my wordcount in a public place, these were sufficient to motivate the competitor in me to buckle down and see the job done.
Emerging with a 75K rough draft in mid December I had learned a lot about writing, my abilities, and the realities of working in a longer story structure. I also had a new favourite writing form.
I'll be disappointed though if I don't manage to repeat the process again this year without the crutch of NaNo. Currently I've scheduled May-June for my next writing project. I know it's possible, but will I see it through?
As for the 'social' aspects of NaNo, I found them a mixed bunch. The forums were often a waste of time, but useful for making my wordcount visible so as to keep myself motivated. I attended one write-in, but found the allegedly social aspect somewhat overrated and/or distracting - it wasn't like we sat around and talked about writing (which might have been helpful), though one of us was a published novelist. I'm a solitary writer, preferring and being more productive in my own quiet writing time at home. The concept of word wars and vomiting wordcount onto a page I generally find abhorrent, so I think (as with anything), you make of NaNoWriMo what you will.
It costs nothing; so if you go into it knowing what you want to get out of it, I can't see how you can go wrong.