I LOVE my spayed mare, she is still sweet but has no behavior of a mare at all. Acts like a gelding. When I first got her last year, she would squeal, kick out at the other horses, squeel at other mares, typical behavior in heat. Peeing all the time, lifting her tail. Got her spayed in November, this year not ONE second of mare attitude. I can lead other horses behind her when she would pin her ears and kick and squeel before.
I have had ALL my mares spayed, about 5 in all. Would not own a mare without spaying her and since there are so many unwanted horses and foal everywhere, that way I don't worry about breeding by accidents or anything.