The Festool-compatible "knockoff" paper is being sold by seller MrZ. In my opinion, the MrZ people are polite and seem to have an acceptable product.
I questioned them about the quality and source of the paper in 2009 when they were starting to sell it on eBay and they offered to send me a few samples. I seem to remember that they sent me around six pieces of 6-inch round paper up to P400 or P600 grit. The paper worked and was closest to Brilliant in its effectiveness.
I am a hobbiest woodworker who does not need to buy 100 pieces of most sandpaper, was debating whether to buy the full box quantities necessary from Mr. Z and then took advantage of a Festool limited time offer and bought the "M0078 ABRASIVES 6 IN. PROMO" from Uncle Bob so I no longer needed to buy from Mr. Z.
Until this year MrZ had the advantage that much finer grit sandpaper than P400 grit was available from him, but now Festool offers these grits in Granat.
My tests with the single disks of MrZ paper showed it to be of uniform grit with no errant scratches. I can't say whether or not the paper outlasts Festool Brilliant-2 or Mirka Gold, but I can say that if I was using large numbers of boxes of paper, that I would have considered it. With the volumes I use, though, I choose to either buy discounted Systainers of paper or Festool paper in quantities of 10 or 20 per grit from the wonderful Festool dealers who break up the boxes into smaller quantities.
Gary