Let me point out that I'm a fan. I'm not sitting out because I don't like it. I'm taking this year off because I'm putting the finishing touches on a book (not a novel) but I plan on participating again next year. I think it's a good exercise in that it helps you develop discipline and find sources of motivation. As a bonus, it's not too shabby for the sake of networking. The breakneck pace guarantees that, in spite of the name, you're not actually going to emerge at the end of 30 days with a fully-crafted, ready-to-publish novel. They tell you that right up front and that's okay. What you do come away with, aside from some level of personal satisfaction for surviving a self-imposed meat-grinder of a schedule, is a giant pile of work product that you can re-visit later, when you have the time to apply nuance, character development, spell check and all the other good stuff needed to produce a novel.
Since I've done it twice, I have two such piles, sitting in folders deep within an external drive. Two great, big, steaming piles of work product, waiting ... on me ... to be shaped into novels. Some day. One of those piles makes a more eloquent case than the other.
"Hey, how about working with me today? I have rich, fleshed-out characters with distinct voices that have stories to tell. In fact, you could probably get an ongoing series of stories out of this. You just need to take time to sit down and come up with a coherent plot and a worthwhile antagonist and finish this off. What do you think? Is today going to be the day?"
I don't say anything as I thumb past, coming across my other pile.
"We got nothin'. My characters are poorly developed and not even particularly interesting, let alone likable. There's no plot to speak of so there isn't even motivation for them to do anything. There are a couple of set pieces that could be turned into something worthwhile but other than that, there's not a lot going on over here. Get to it when you can, I guess. It's not like we're goin' anywhere."
I don't even make eye contact with that pile.
The beauty of NaNoWriMo is there are no penalties. If you come up short on your word count, nobody yells at you. If the end result in terms of work product kind of sucks (and it probably will), there's no harsh critiques. Nobody judges you in any way on any of it. It's like being hired for a tough job with no consequences for failure that you quit after a month and get to keep everything you worked on. It's the perfect combination of the sense of urgency that comes when working against a deadline and not really having to worry about anything. I may never get to either of those massive piles of words but they're there if I need or want them.
Next November, if you've got a germ of an idea that requires you to just sit down and write it already, you could do worse for yourself than running it through the NaNoWriMo process.