Have one and love it, BUT (isn't there always a but?) do all of your cutting with it. This ruler more than others will cut just shy of the width wanted.
To help relive this problem I hold the ruler right next to the slot I am cutting. (Please be very, very careful that the tip of your finger is not over the slot----ouch!) If you watch the slot when you cut you will notice that the ruler moves a little each way next to the cutter. It's hard to explain unless you use one. I think it happens if you don't hold your cutter straight up and down. I tilt my cutter a little at times.
The 'v' in the center of the fabric isn't from the ruler you use, just the way you fold the fabric. After pressing I hold my fabric by the first fold and 'shake' it. Lay it on the table and fold again lining up first fold with salvage edge. This has stopped most of my 'v's'....