I'm doing it now, and my fictional theme is infidelity related. That's what grabbed me enough to motivate me to write 50K words in 30 days, and since I've become rather an expert on the topic over the past 3 years, I didn't have to do nearly the amount of research!
Tonight I should hit 40K, and I'm so sick of my characters that I can hardly stand them! My W wrote her 1st novel last year (not NaNo), and she said the same thing happened to her too.
Anyhow, I think NaNo is an incredible exercise in persistence, and I recommend it to anyone who enjoys writing.
The one thing I would say is to decide on your topic, think about your characters, and perhaps write an outline before you start on Nov. 1st.
ETA: I think my novel SUCKS, but at least I'll finish it! DDay 7/4/07 found out about online/sexting EA with OM
DDay 7/25/07 found out about OW
2 DSs, ages 8 and 6