Gary,
I've had one for a while and it sure does make it easy to get exact duplicate spacing from one side of a cabinet to the other. I made a little cabinet for the shop with six drawers, and used it to layout the location of the rails and the centerlines of the drawer slides. I didn't have enough tabs so had to do the layout in two steps - rails and then slide centers - but it gave me an exact mirror from one side to the other. I've never used it to measure inside/outside dimensions as they show in their videos - I have the Lee Valley Bar Gauge Heads
Veritas® Bar Gauge Heads - Lee Valley Tools.
Screwing the rails in place using a block clamped to the layout line to stop it from moving. You can see the lines for the center of the slides as it was easier to lay them out before installing the rails.
With the rails in place and the top and bottom attached. Then it has to be flipped over and set down on the second side - it's a little tricky picking up the first side with the rails attached and then dropping it down on the second side, I had to have a block clamped at each rail location and a bead of glue on the face of the side - and then work fast.
All the drawers installed. I'd laid out the center of the slides on the drawers, screwed them to the drawers using the slotted holes so I could fine tune the spacing once the drawers were in place.
Typical Woodpecker quality, well designed, and worth every penny.