I don't know if anyone else has raced with them, but Duff Beer (Charles, Ricky, Robert and me) went to AMP last weekend and raced in their GP4 class. Only 3 cars showed up for GP4 out of 22 cars overall.
It's an 8-hour race on Saturday and Sunday, separate race each day. They only want experienced drivers (Solo or Instructor level or extensive W2W seat time) and class by power/weight (GP1 through GP4). There is no value requirement, so bring what you got.
They are extremely contact-averse, which we liked, and the driving was very good. One car did have a brake failure and smashed in to another car, destroying one car and seriously damaging the other. Both drivers were fine.
We won the Saturday race and came in 3 laps behind an E30 (ETA-engined 325E) on Sunday. As an alternative to the Chump crash-fest, I'd like if it had more cars in the field, but we'll be racing with them again.
http://www.racewrl.com/