It's a nice idea, but if they get soaking wet or on abrasive surfaces, it tends to come right out, the best way to hold it in is with a shoe,or boot, ironically. WHile it "sets" in 30 seconds, that's in ideal temperatures and humidity, seems like it takes a bit longer in reality. And, it really holds in the moisture, and that leads to some thrush. Just pulled shoes off a mare that had it in there because the guy that trimmed her on my maternity leave sored her, so he shod her and used this to cushion her and her feet smelled ROTTEN when I pulled it off, and the frog sloughed off. That's a worst case scenario, but the boots, though a pain in the hiney, are still a more durable option, plus, you don't have to hold up their feet for minutes letting it dry.
After all that negative review, it IS good for very temporary cushioning for horses just coming out of shoes, but I wouldn't expect it or want it to stay in 3 weeks straight. Good in founder instances to support the sole temporarily, nor flat soled horses that are going to be turned out, but I woudln't count on it to stay in for riding, unless you put boots over it. I think its' best application is for horses that would need pads inside the boots until they adjust to not having shoes or special needs horses, and it just frees up the owner to not deal with loose pads, but it would need changing and would be weighed against the thrush possiblity. They also make a "anti thrush" sole pack, but chemicals aren't going to fix the fact that the lack of air is the problem to begin with.
And, it's a bit more expensive than just using boots. Just my thoughts on it. WOuld be intresting to see what you find when you try it.