I had a morab mare for 14 years. I got her when she was 15 mo. Old, and she wasn't even halter broke. She turned into a great riding horse, but her marish behavior was really frustrating sometimes. She was always aloof, and could be very moody when she was in heat. I think some of it went back to her lack of human interaction when she was young, but a lot of it was also just the hormones. Last year I finally sold her (to someone I know, and they take great care of her), and life has been peaceful!
My current horse is a Friesian cross gelding. He's almost 3, and I've had him since he was 4 mo. Old. He is just the exact opposite of my mare. He gallops to me from out in the pastures everytime I arrive at the barn, he'd rather be with people than the other horses (although he likes them too), and he's just been such a joy to train. One difference between Cody and my mare is that Cody was imprinted as a newborn and interacted with humans every day. However, he's also not dealing with the extremes in hormones--it's so nice to not have to worry about him going into heat
I know some people who swear by their mares, and I'm glad they've had good experiences. For me, I wouldn't buy another filly unless I thought there was something pretty exceptional in its temperament. JMO though :)