I have one of these feet, but I don't use it anymore because I'm too much of a perfectionist for SITD. What I am doing instead now is using a "serptentine" stitch, which is basically a wavy stitch to left and right. It goes right over the seam allowance but I no longer have to worry about deviations from the seam (they basically don't show up) plus I can sew faster this way.