I've heard that it wears away fairly quick. People have sprayed their shoes and stuff and it's waterproof for a while but then it wears down and fails down the road. Considering how expensive it is to coat shoes and stuff with it, I'm not sure it's a great long-term solution. But, the stuff is neat and really works if you spray a lot of it. YMMV, of course.
One of my main concerns with the stuff is (AFAIK) it hasn't been tested by a third party for safety yet. If I used that stuff, I would wear protection for my lungs and perhaps skin as well. I doubt breathing it in would be fantastic for your lungs.