I've fed it. Horses love it. It's basically somewhere between hay and grass, moisture content wise, and it's fermented.
Good haylage is pretty tricky to make, has to be stored right. Any damage to the plastic wrap leads to mold and bad bacteria in the bale. It can cause botulism if an animal carcass happens to be in the bale. My hay guy used to make round bale silage, stored it in a sand bed with protective netting over it to prevent animals from damaging the plastic wrapping.
It is very nutritious, dustfree, ideal for dust- and mold allergic horses.
But a bale should be fed as fast as possible, shouldn't sit open for more than a week in winter, 2-3 days in summer.
If you try it, switch over slowly, just like with new sweetfeed or pasture in spring, otherwise you'll have horses with diarrhea.
Watch closely what you feed, white spots mean fungus, discard. Dangerous. If it's warm, discard.
It's tricky. I was too chicken, but im a worry wart. People I know have been feeding it for years without any problem