I have used it on my last two purchases. It is well worth it to get the month long service, since it's only a few bucks more than the single report. I found a few things here and there, like the one that had 3 accident reports in 2 years. Granted, they may have all been minor fender-benders, but I still didn't want one that had been abused like that. It also helped me avoid a couple of cars that had been on the Gulf Coast in Aug 2005 when Katrina paid a visit.
As someone else pointed out, you still probably want to take any vehicle you are considering to be checked out. Reporting accidents and insurance claims is optional, as long as the title status isn't affected. (Salvaged, stolen, etc) A car could be in many accidents, and you might not know it if they were never reported to Carfax.