I have one and dislike it, so I never use it. Even if it were accurate (which it isn't), I would have to keep my eyes glued to the edge of the foot in order to get straight seams. I prefer instead to use an adhesive guide as a physical barrier against which I butt the edges of the fabric. This is much easier on the eyes, plus I can sew much faster and still have a straight seam. I have used moleskin for this, but currently this is my favorite:
http://www.amazon.com/Dritz-Quilting...dp/B0085L2Y2I/