I'm in a small writing club and all four of us are trying it. I successfully finished it two years ago with a series of ten interlocking short stories that (surprise!) tell a complete story as a novel by the end. One thing I notice is that a lot of people use it as a project to just get words out, regardless of quality. They talk about vomit-drafts, and/or just sitting down to write to see where it takes them, to see…
An analogy I would make is that a marathon is measured as 26.2 miles. It's not measured in minutes or hours -- it's not the 2:30 run, for example. The standard of success is measured in miles, not minutes. Similarly, NaNoWriMo is measured in words, not quality or twists or ready-to-be-published state.
You are right, though. It's good motivation. I've finished every year for 10 years, but I've never gotten anything to a state I would consider to be even remotely publishable. :)