I've done this the past few years, and I have to say that it is an incredible experience.
I started out in 2006 as someone that has fallen in the trap of developing the same world and story for so long that all other ideas had been more or less pushed to the side. October came, and I heard about NaNoWriMo from a friend that had done it the year previous. She challenged me, and me being me, I couldn't back down. So I looked through my 'Ideas on Hold' notebook to see what I should pull out. I thought that since the spirit of NaNoWriMo was to write something quickly and feel the flow of an evolving story, I would pick an idea that was fairly vague.
So I selected my host of characters, which at this point only included the protagonists (The Guardians of Tralos), the king, and a few prototype dark mage types and that my keyword themes were "Conspiracy" and "Treason" (which helped spawn the title, The Tralos Conspiracy). As I wrote, the plot unfolded into a whole lot more than I thought it would be, and found that by the time I had reached the end, something completely different had come out of it. The "Conspiracy" and "Treason" were still there, but neither were what I thought they would be, and the story was left, by novel's end, unfinished. Oh, and it was a NaNoWriMo victory, too.
I did it again in 2007, hitting the 50k word line early, but this one did not have the same surprise twist for me that the previous year had. Part of it was that I decided to go with the sequel to The Tralos Conspiracy, which is The Tralos Conflict. I'm actually still putting the finishing touches on this one at around 100k words, so it's short, but since my writing is still developing, I figure that these drafts will serve mostly as a guide to when I can come back and write them 'for real' (for fun, of course... this is merely a hobby for me).
This year... I'm not sure if I will be able to hit the mark, but I'm going to try. When the Tralos Conspiracy and Tralos Conflict failed to be resolved in two books, I fell into the 'trilogy trap,' so with a little luck, I can finish the story and score a third NaNoWriMo victory with The Tralos Crisis.
Sorry for the length... I didn't intend to go off on this tangent when I first started this post.