Spray On Waterproofing

It's definitley interesting. But I seriously doubt if it has any kind of breathability.

Neverwet has been confirmed by its creators to not have any effect whatsoever on breathe-ability.

It also won't wash off.

Not much in the way of negatives! I doubt that a product currently available at a shoe store is really the same thing, though. Hydrophobic chemicals can be designed very differently, and I haven't seen anything else out there that claims the same properties as Neverwet. Mind you… it's not out yet, either, so it remains to be proven.

The stuff at shoe stores as far as I've seen are a silicone spray. AND THEY SUCK! Almost got frostbite while hunting with a pare of "treated" boots. Use a wax based product to waterproof your shoes,if they are leather. Best waterproofing I've ever used.
Of course, if you want something for fabrics you're out of luck. Unless perhaps the spray stuff would work better on garments that won't be submersed in water like shoes? I've never tried to spray a shirt or anything. Why don't you do that for us. Spray a t-shirt and go stand in the shower and let us know how it goes. Then wear the shirt for a couple days and see if you get noticeable less breathability.

As for me, I just hope that neverwet stuff holds up to its claims. It sure will make me happy, even outside of backpacking applications.