I installed the Challenge axle back about a week ago. It sounds incredible in my opinion. I am coming from an Eisenmann Sport paired with the Challenge Street x-pipe. I am running the stock x-pipe pipe with the new Challenge exhaust and I think it would be too loud for my taste with an aftermarket x-pipe. I will post a full review soon. Very happy so far.
2013 E92 M3 DCT/LMB/Blk Nov/Competition/Premium/Cold Weather/EPS/BMW Apps/SAT Radio