I just started FMQ and just started learning my new machine and I love the S/S button! It is too difficult for me to maintain a steady speed with my foot pedal. Now my stitches are so much closer to the same length and it is so much fun. It's a Janome too, and it seems so easy to use.
I worried about starting and stopping but realized I can just sit in one place which ties the stitch to start and stop, and this is required anyway. I can also sit in one spot for a second to hit the stop button and it won't change the design at all!
I forgot to mention I go pretty slow, my speed is set about 1/3 of the way from the left, and this helps me to relax and move the direction I want without panicking.