I've used it and didn't like it.
You have to be very careful with knots, if you read the bumph that comes with it it says that it works best with one particular knot, truth is it don't seem to work with any others.
I believe that there are better products around John, I've been using Drennan Fluorocarbon Fly Leader in 8lb bs for hooklengths and found it works just fine with either a Grinner knot or a Palomar knot.