Great guide for someone who is starting out with whetstone sharpening. In case if anyone is curious- this is for correctly sharpening a Japanese angled knife (mostly for kitchen knives, I would think) as the angle is designed for Japanese knives, not Western knives. With this however, you can potentially still sharpen your other knives, but you'll be at it for a long, long time to thin the edge to this angle.
I feel that this is like the knife-sharpening equivalent of training wheels on a bike. I think the intention of this is so that a student can really learn and feel the correct angle for sharpening, and once they are proficient they would no longer need this item. This is very secure once placed onto the knife blade, and the ceramic edges are very smooth and glide across the whetstone easily.
As someone else noted, this did leave a couple of scratch lines on the top of my knives, but these are cheap old knives so I dont really care. However, I am not sure I would be so ambivalent if I had scratched my Damascus Shun knives... I would be extra cautious when placing on a nice, shiny blade, and would thoroughly rinse both the blade AND the guide in case you have to reposition it - as i think the scratches were due to having a bit of grit from the whetstone inside the guide when moving it down the blade.
Overall though, I think this is very useful for a guide to learn what angle to hold the blade for correct sharpening - I will keep practicing on my cheap knives until I am able to correctly sharpen my nice knives freehand.