I have the 15-91 and I love it. I use it mostly for chain piecing with a 1/4" foot.
I have tried it with the FMQ foot. It is really easy to lift the machine up and loosen a screw drop the feed dogs. The bobbin size is HUGE... I really appreciate that. I also enjoy the large size of the sewing table.
The one issue I had was the presser foot had a little too much pressure. I think you can turn the knob up top to reduce pressure, maybe someone can confirm this?
I got my fmq foot from sew-classic.com, they have nice machine parts!
I have been spoiled by a quilting frame and a 17" Voyager, so I only did the one project FMQ on the 15-91. More throat is better for full size or queen sized quilts.