Hopefully this doesn't get buried, but....
I created an anonymous GRE vs PowerPrep Score survey to track whether PowerPrep scores are a good indicator of real GRE scores. If you've taken at least one PowerPrep test and the official GRE please fill it out for future GRE sitters. If you are curious about the results, here is the spreadsheet of all the responses. If there are any issues or questions, please don't hesitate to comment or PM me.
Sidenote: Does anyone think asking for SAT scores would be useful?
My data
PowerPrep 1: 167V 169Q
PowerPrep 2: 169V 170Q
GRE: 170V 168Q