I've tried this stuff on a wide variety of things from bivvy bags, fabric shoes and of course waterproof shells.
I've followed the instructions to the letter, used their tech wash first etc. I've tried the wash-in one and the spray-on on.
I've tried handwashing, I've tried machine washing.
Every time it looks like it's going to be great, water beads under the tap like it's quicksilver. And every time it meets British weather, it's useless. Wets out in about 1/2 an hour.
This weekend I walked in the Brecons. It was pretty damp with rainstorms and a Sunday pretty much full of constant blown drizzle. My Helly Hansen jacket's repellency survived the first downpour and beaded up OK but after that it was all wet out. Jacket is nicely waterproof and the only dampness was under the arms and my back - clearly sweat. I'd only just washed and treated the jacket.
I'm trying to work out if there is really any point at all in it. I'd previously used Grangers Xtreme Repel on a bivvy bag which, again, Nikwax was proving useless on. That seemed better but they've withdrawn it now and only old stock remains. I believe it was a C6 emulsion.