I just bought some of this from the boys at Tarty as I couldn't be bothered hunting out tar. I've always just used smooth rims and currently run the magura black pads (not for much longer). I bought some thinking it may work it may not and applied some to my rim today and had a whirl...WOW! I couldn't believe how much the brakes locked up, I think I added too much but the performance increased 10 fold...so solid!
I dread to think what a grind and some decent pads would be like!
1 happy consumer!