I use the Stop/Start button for some of the fancy stitches on my Janome, button holes, and for bobbin winding. I use it for some of the fancy stitches because the speed is more consistent than my foot control is with them! Also, when i went to 'stop' a completed run of fancy stitches, I know it will stop following a completed design. I have not used it for regular straight/zigzag etc.
I have not had any problems with it.