I've had some really wonderful experiences with mustangs. I love them, and I look forward to adopting and starting one someday when I have the time/money/space for another horse. I've had the pleasure of experiencing one of those extreme bonds with a mustang, and I've witnessed a friend do it too with hers.
It sounds like you're doing a great job with your filly (love her first ride, btw!). I know mustangs have a bad rap sometimes, but honestly many breeds do. My totally sane and mild-mannered OTTB and I are trying to change OTTB perceptions one person at a time, for instance.