I mentioned them to my farrier the last time he was out (as a joke, I wasn't being serious, my horse is barefoot) and there was some mention of pigs flying and the day I bought a plastic horse is the day he'd nail on a plastic shoe.
Horseshoes should be shaped to fit each individual hoof, not the other way around. You can't change the shape of a pre-done plastic shoe. My farrier's biggest pet peeve is when someone takes a metal shoe and hacks away the horse's hooves until they fit the shoe, instead of changing the shoe to fit the hoof. I heard all about it as he was yanking the shoes off a recently purchased pony and trying to salvage what was left of the poor thing's hooves.