I have used CarFax quite abit. A few years ago my daughter bought a used veh from a dealer & did not have me check out the car via CarFax. She had asked the dealer to provide one since most dealers will hand you a complete CF if asked, but this dealer ignore her request. After having some immediate problems with the car, I ran the CF & discovered that it had been totalled in an accident, and a Salvage title issued. After several phone calls and the suggestion that attorneys get involved, they took the vehicle back and found here one that had a clean history...she is still driving that one and happy. All of that could have been avoided if she had waited for DAD to do some homework.
Cf is not the last word on potential problems since many things can happen to a car that never gets reported. But, for the few dollars it take to run the VIN, I think the benefits are worth every penny.