Well, I tried it the first year, and reached 100 pages, but I wasn't really satisfied with the results. That first year, the month they chose was June (to coincide with the Summer Blockbuster season), and NaNoWriMo was a distant enough memory that I had gotten my courage back by that time.
The second year, they moved it to April (so that they could get schoolkids involved in group scriptwriting). And that's just too close to December, for me. So I couldn't find the stamina to write any more than "Act one, scene one" on the first page.
This last year, I skipped NaNo, and The Art Garden is on hold until further notice, and I was beginning to feel the itch for organized writing.