I did it the past four years, and gained a lot of techniques and process that have helped me in my full time writing life. I went beyond the 50K each time. I mentored for three years, too, which was a mixture of joy and frustration.
I can't do it this year because of paid, contracted deadlines and constant travel. I also realized that I write more than 50K in any given month -- it's just not all on one project.
I miss it, but this year it didn't make sense to do it. Maybe I'll get to do it in a future year. Of the four projects I wrote, one is under consideration, one is almost ready for submission, and the other two are in rewrites.
anything done in Nano, I found, needs far more revision that my normal pace, due to the emphasis on quantity instead of quality.