I really love the 15-90's. I've only had a few though - but so far I like them better than the 91's. I get better speed control with the external motor - and I like that for FMQ.
Even though I have a long arm, I'll keep a 15-90/91 around for smaller FMQ and touchup work. You know- for when after you get done and take the quilt off the frame - and then while working on binding, you find a spot you missed! I'm sure the recipient of the quilt would never notice - but it still bothers me.