IMO, from my experience, the Mustang is better than the stock OEM seat, but that's about all I can say about it. My wife was uncomfortable with her Mustang seat as well.
We both now have Russell seats on our 900 Classics and are very comfortable now. The Russell is more expensive than the Mustang, about $200 more, and worth every penny of it, too. It's a truly custom made seat and they make it exactly to your needs and specifications. I don't know where you live, but it's really worthwhile to arrange for a ride-in appointment to have your seat made because you're right there for the fittings. Of course if you live on the other side of the country that wouldn't be very convenient. We are fortunate to live just a day's drive to Redding, CA.
"Don't sweat the small stuff---it's all small stuff"
'09 650 VStrom