Contrary to what it says on the bottle, I suspect this requires a cycle in the dryer to activate the waterproofing and improve effectiveness.
Otherwise, it works a treat, and gives your jacket a super-smooth surface for water to bead off.
1 pass of my North Face Goretex jacket (factory waterproofed when new) with this stuff was enough to keep the water beading off the jacket for a week's worth of rain.
When I say a week's worth, I mean it was over 2 weeks with intermittent rain almost every other day (walked in the rain for about 40 mins per day); so some of it might rub off when you're going about your day's business, thereby wearing off.
I also went a step further and coated the top of my backpack with the stuff as an experiment; it slowed the ingress of water into my bag enough such that I could get home without having wet paper edges. It wasn't as effective because the material my backpack is made of is much coarser than goretex - but it still works nonetheless.
I then went a little bit crazy and coated my Crumpler camera pouch (chicken tex fabric), goretex hiking boots, tripod carrier........