When I tried using it, I discovered that it doesn't have a locking device to keep it from moving. If you are not careful, you can be off a bit and the next line you will be off a bit more. When marking with painter's tape, I always refer back to my original center line to make sure my lines are consistently straight. HOPE that makes sense. You can also use what is called a HERA marker by Clover. It just makes a crease in your quilt top. JAF has them in the quilting section and Eleanor Burns has used it on her show. But again, you need to make that center line and always refer back to it.
http://www.ebay.com/itm/Clover-Hera-...item19f3d75be2