I think NaNoWriMo is terrific.
To my knowledge, I only have one NaNoWriMo novel on my list so far, though it's entirely possible there are two or three more that were originally composed in the month of November, and the author's just never told me the novel's full origin story.
Here are some things that are great about NaNoWriMo:
1) For lots of people, it's the excuse they've been waiting for to finally get off their duff and write a novel. Many, many first drafts of first novels have been composed as NaNoWriMo projects, I'm absolutely certain.
2) For lots of people, even published authors, it's a chance to make a huge and productive start on a new book project. It anchors the writing calendar. NaNoWriMo is a chance to produce what Anne Lamott calls a "sh*tty first draft," to lay the story out on the page so that you have something to work with.
3) For lots of people, it provides a sense of discipline. I look sometimes at the Twitter feed for the hashtag "#amwriting" and think to myself, "you're not writing, you're tweeting." I think the 50,000 word count is an ambitious and useful goal, a reason to disconnect yourself from the internet and really buckle down and write.
However. I see an increase in my queries every year come the first week in December.
I love NaNoWriMo. One of these years, I'm going to get off my own duff and do a NaNoWriMo project, just for the experience.
I don't love unrevised NaNoWriMo projects. Those proud NaNoWriMo finishers who send me their novel the first week in December haven't even had enough time to reread their manuscripts before pushing send. This is a big mistake.
Take the month of November to write the novel. Take the month of December to let it marinate. Give yourself a little psychic distance.
Then, in January, after you've put the holiday decorations away (or not. I put mine away in February this year; who am I to judge?), get out the manuscript and read it with fresh eyes. Read it alongside a book on how to write and revise. There are tons of great ones out there.
Make yourself a list of the main issues that need addressing in the revision. Some major things you should be looking for:
-plot: is it working? in a romance, do the hero and heroine end up together? in a mystery, is it too obvious or too subtle who the bad guy is?
-characters: what are their motivations? are they consistent? are they realistic?
-dialogue: do your characters talk like real people? Are their phrasings age-appropriate and in keeping with their characters? (and how are your dialogue tags?)
-pacing: does the story hold your interest throughout? I'm especially obsessed with the "flabby middle," which I'll talk about more another time, but in short: many novels have a slow spot right around the midpoint. Fix this.
-genre: this reread is a good opportunity to start thinking about how to market your novel. Who do you think is the target audience for your book? (Please don't say "everybody.")
Once you've got your list, give yourself at LEAST another month to do the revisions, and lather, rinse, and repeat.
Then, if you're very happy with the manuscript in its double-revised form, start the query process.