I have a Circle Y Flex tree trail saddle. At first it didn't seem to fit my wide mare that well, but then it seemed to adjust to her. It is a great saddle. It is very comfortable and cushiony. If I had a criticism of it, it would be that it comes up a little too high in the front, making forward seat positions ackward. As far as slipping and turning, it doesn't. I put it on a horse that is thinner than my horse. I didn't realize that the cinch was not snug enough. After riding, I realized that the cinch was hanging so loose that you could easily put two hands under it. The saddle stayed right with the horse. Granted, the horse does have nice withers, but it still speaks well for the saddle.
I am not using it right now because I am using an endurance saddle just because it is so easy to throw up on the horse.