1) You write a friggin' novel
2) Take a look at 1) again. Really soak it in.
3) The creative freedom afforded by hogtying your inner editor is a wonderful thing. Really, just cutting loose and letting things happen without worrying about future consequences or even such things as readability is an amazing experience and can lead to the most awesome insights to your own abilities.
4) Did I mention that you write a novel in a month? Novel. in. a. month.
1) NaNoWriMo takes a lot of work and on top of that, it doesn't give you a material reward beyond the novel itself. No ten grand or shiny trophy. Poopy.
2) Remember sleep? Your best friend and ally against the crazy and the cranky? Yeah, say good bye. You won't be seeing each other for a while.
3) Your novel (y'know, the one you wrote in thirty days, losing valuable sleep for, etc. etc... yeah, that one) will most likely be crap.
It's a first draft. All first drafts are shit. So that amazing novel that you pounded out in thirty days will suck That's because the odds are like a bajillion to one that you would write a publishable, or even completely readable, novel in thirty days.
See, the thing is, first drafts are supposed to suck. We have to let them suck so that we can use that suck as a vacuum to pull all the little niggling ideas that haven't really been developed yet out of our brains and into the collection bin of scribbles. My first drafts are always filled with unusable subplots and weird character moments and long scenes of nothing important happening. Very often, my timeline changes and huge chunks of my novel are deleted in editing.
But that's okay. An idol of mine once said that good novels are made in revision. First drafts just get all the nuggets of blahsome onto paper and then editing is the "bl" out and just leaving the core nuggets of "ahsome", which we then edit until it becomes "awesome".
All in all, for me, the idea of a novel in a month outweighs even the idea that I might spend months more editing the stupid thing.