The price isn't too bad. While it looks like the jig accurately cuts dadoes, it doesn't register the location of the jig vs the dado/shelf- you still have to set the fence somewhere.
A better jig from one of the WW mags is one where the router rides on two pieces of tempered hardboard or the like, which each have a hardwood fence. You build it so the hardboard sticks out slightly more than the distance from the edge of the router to the bit on one side and wide enough for a second bit and clamping area on the other side. To calibrate the jig (only needs to be done once per side per bit) you run the router against each fence, which trims the hardboard exactly to the edge of the bit. From then on, just clamp one half of the jig on one of your layout lines, put a piece of shelf between the hardboard pieces, snug up the other half and clamp it. The edges of the hardoard always show exactly where the bit will cut.