I think NaNo is like junior varsity sports - work at it, excel, and the most you'll probably be is a good junior varsity athlete, which is to say, no particular kind of athlete at all. You learn all over again when facing a more serious task. This is my second year doing NaNoWriMo, after sputtering out at 35,000 words last year. I'll finish this year's novel for certain; it's a strong idea and I'm putting real care into it. But 'real care' is anathema to most WriMos, and I think the less affinity you feel with the culture/philosophy of the thing, the more likely you are to get serious about the writing.

It's not a bad way to find out whether you're capable of finishing large-scale writing projects (other than academic theses, which have their own motivational apparatus), mind you, but the NaNo aesthetic is tiresome, and I think the culture of NaNo imparts bad lessons - as would any subpar writing group, say. If you want to write a novel, just write a novel.

NaNoWriMo's philosophy seems similar to that of Nora Roberts -- "You can fix a poorly written page, but you can't fix a blank one" -- and while her novels' literary value isn't high, her resulting net worth certainly is.