I've done NaNoWriMo for six years now. Unfortunately, I think this year's now-aborted attempt will be my last.
First, I am never into my stories enough to see them through. That's a major contributor.
Worse, though, is the toll that NaNo takes on my relationship with my husband. Even though I'm a fast typist, I am a super slow writer, so I use every non-work moment to write during November. That means I stay out writing until midnight, come home and crash, and repeat 30 times. Or else I go straight home after work but hide out away from him, writing by myself. Either way, we don't really interact when I'm doing NaNo.
It would be different if I had six novels to show for it. But I don't. I have one zeroth-draft from the single time I "won" NaNo, five drafts at various stages of "completion," and no real desire to go back and spent the time and effort to work on any of them.
This doesn't mean I'll never try writing novels again. I look longingly enough at published books and have story ideas wander into my brain often enough that I'll try it again, under less time-crunch-iness. But I don't think that the format of NaNo is working for me — no matter how much fun the community spirit is, unfortunately. I'll miss the camaraderie and the zaniness, but overall it's just fitting into my life anymore. :(