I am putting my mare on one for winter, we ride less and there isn't any graze so she tends to get bored quicker. Started using it a few days ago, a nylon one with smaller elastic holes so she has to work to get at it. It works really well! I love it and kept her busy eating her hay meal for a good three hours, three times longer than usual.
I have a feeling that she'll find some way to break it open eventually... kinda quirky like that. But from limited personal experience- I'd say hay nets are one of my favorite inventions.