I bought a 15 about a month ago specifically for FMQing. It is wonderful!!! It was in a gigantic, old cabinet, and I put some wooden tv trays along the back of the machine and I have a super surface to work with!

I have done FMQ with my Singer 15-90 and 237 treadles with no problems at all; they have had motorectomies done.
I don't have much luck with the Singer 201 and 66-1 treadles, these have the drop-in bobbins; the thread path is less direct and that seems to cause problems with FM. I love piecing with them.