I have some customers who have used it so I'm familiar with what it is.
Also, contrary to one of the answers, mineral wool is not the same thing at all as 703 or 705. One is made from glass fibers, the other from mineral slag. Besides density, it is a different material, has different gas permiability ratings, and a different matrix makeup.
For instance, compare...
Roxul Mineral Wool
RXL 60 3" 6.0 pcf (96 kg/m3) 0.78 0.89 1.04 0.98 1.01 1.02
OC Fiberglass
705, plain 3" 6.0 pcf (96 kg/m3) 0.54 1.12 1.23 1.07 1.01 1.05
Same thickness, same density, same 'A' mounting tests - very different results - expecially at 125hz. Yes - results vary from lab to lab but not by almost 40% - not if the test is done properly. In addition, the mineral wool is 'springier' than the 705 which is stiffer. So, if you add a scrim face (FRK/FSK) the membrane effect will be different and at a different center frequency.
Now, throw in 3" 703
703, plain 3" 3.0 pcf (48 kg/m3) 0.53 1.19 1.21 1.08 1.01 1.04
As you can see, 703 and 705 are much closer to the same thing than 703 and the mineral wool are.
All have their place - just a matter of what the room needs
Bryan