I made them, via the Woodsmith plans back when they were free. They are terrific. I made 4, and fudged the size by an inch or so, so that 2 stacked = height of my MFT. They are super sturdy, I used them to make my split top roubo, and they didn't tip/tilt/wobble....and these were 8 ft/6inch thick slabs.
When I need to get to the top of my lumber rack, i stack them and climb up. Never felt like i was wobbly or taking excessive risk.
Finally, the lower crosspiece acts as a shelf. Good little storage spot.
One thing I would do differently: I would have rounded over the edges on the feet, pushing/pulling them across my epoxy (nonslip) has caused them to "flake" a little on the plywood edges.
Are they a good value for the price of plywood + time? For me, yes.