I've been participating in NaNo since 2005 - I'm a winner for 2005, 2007 (missed year 2, just like everybody else
) 2008, and 2009. I couldn't participate last year due to life commitments, unfortunately.
Honestly, it's the only reason I started finishing stuff. I work really well under a deadline, and the 30 days is a good challenge for me.
I'd like to participate again this year... it will depend on where I'm at with other projects when November rolls around. I love NaNo - the whole atmosphere is so supportive and festive. It's a really great challenge, and the community is just wonderful.
If you're looking for a powerful writing challenge, I highly recommend participating.