I checked on a car that I was selling. The Carfax teaser claimed that it had "5 important vehicle records". It turned out that the "important" records were when I registered it (as the 2nd owner), and various smog checks. I complained to Carfax that smog checks were to be expected, and not "important records"> I asked for a refund under their "money back if not satisfied", and they refunded my money.

I think it is worth doing if you're buying a car, especially on the east coast. (It just seems like odometer rollback is more common there than on the west coast.)