I've never seen it used for the whole roof on a barn, but have seen it for skylights. I have an aquaintance that used it to build a greenhouse, and it's pretty toasty in there. I'd be hesitant to use it for more than a skylight on a horse barn.
I used it for a run-in roof attached to the end of my barn. It's great in the winter, nice, warm and sunny...but it does get hot in the summer, and I have three sides open then. The area I covered is about 16' x 16'. I wish now I had gotten the white version instead of the clear. One mistake we made was to skip siliconing the panels down in addition to screwing; after 2 years we've had to reattach the panels the wind has loosened, and I believe that silicone would have helped.
ondura roof panels are pretty much the same price, same location, but are made like asphalt shingles, just in a large corregated panel, like the pvc panel.........all you need to install are the special nails.....they are easy to use, move, and very durable..........i have them on several things here in pa, and they have held up for decades