So, I have to confess: I'm not writing entirely new content for Nano. Instead, I'm using Nano to motivate me to edit my existing content into a coherent rough draft. As a result, I'm about to turn the 50K word mark, but I've still got significant editing to go. And I am writing new content - writing the scenes I hadn't written, filling out scenes that are too brief or need more development, etc.
Still, Nanowrimo has benefited me tremendously. For one thing, it's forced me to read through my novel quickly. I've been sitting down and reading through large chunks at a time, which helps me to start getting a better big picture view of my work. Another benefit is the fun of it. I am participating in an event with other writers and we all get to complain and rejoice together. What fun! As I continue to move through this month, I expect to see other benefits crop up.