The only time I've used the start/stop button is when I'm sewing certain decorative stitches - no idea if this is standard or not but on my machine if you use the start/stop button, it'll finish sewing a complete motif before it actually stops, no matter when you hit the button.
Other than that...I can't get used to it at all. I'm a pedal-er. I couldn't get used to the knee lifter either, it just got in my way so I took it off. I would really hate to use the button for FMQ, I need to be able to floor that pedal when I go around curves!