Check out this report from The Washington Post regarding Carfax's shortcomings - not to say it isn't a valuable service - just know the limitations

http://blog.washingtonpost.com/thecheckout/

 

I hate them, I once signed up to see why a car was salvaged, all the report said is that it was salvaged which I already knew. I wanted to know the reason it was salvaged which is what carfax told me it would do. Needless to say I had to call my CC company to get the money back.

 

I use carfax primarily to eliminate cars with accident/odo tampering history. A clean carfax requires interpretation as it does not guarantee anything.