National Novel Writing Month, like most everything else, has some vocal detractors; I am certainly not one of them. I know several professional writers who have used the encouraging support system of NaNoWriMo as a way to jump-start a new book or power through a work in progress. The fruits of their labors can be found in a bookstore near you.
But even if the average NaNoWriMo novel doesn't see professional publication, I think it's an entirely worthy endeavor. It cuts to the basic necessity of a writer: writing. It's time to stop making excuses and start writing that novel you've been talking about for years. And not just write it, finish it. Even if it sucks, you can say you finished a novel. And next time? It'll be better.
Even if an aspiring novelist becomes no better at his or her craft as a result of putting 50,000 new words on paper (and a total lack of craft improvement is hard to imagine, really) and becomes no wiser, he or she will surely come away with a taste of what it's like to labor as a professional writer. The awkward schedule juggling. The late coffee-fueled nights and sacrificed social calendar. Never discount the value of learning how to work creative productivity into your daily routine.
If a NaNoWriMo participant is lucky, he or she will experience couple of magical evenings when the words flow seemingly of their own accord and the story transcends all those story notes and character sketches and becomes something the writer never imagined being able to create.
And that's something we all live to write for.
As a respectful response to BookReader's writeup below ... the premise is not flawed. A working novelist may very well end up with a crazy deadline of having to write a novel from start to finish in less than two months. It happens. Midlist authors are frequently broke and freelance novel work is a feast-or-famine proposition. Authors get calls from their agent saying things like "Big Publisher has a gaming novelization that Big Name Author just bailed on ... can you write a book from this guy's outline? I'll email it to you. The completed novel is due in four weeks, and they'll pay you $10,000 on acceptance."
I've gotten that call, and I had to decline; I'd just started a new day job and didn't have time to drop everything and write. I still wonder "what if?" about that one. But if you are in a position to drop everything and write, and if you absolutely, positively need to make your rent that month, the correct thing to say to your agent is "Absolutely, yes, I can do that." Because if you can produce under those crazy deadlines, more work will assuredly be coming your way; if you punk out and fail, alas (and sorry about your professional reputation, buddy). And having been through NaNoWriMo a time or two is very good practice for this far-from-ideal but actual real-life deadline scenario.