I haven't used them but I just watched a U-Tube video and they seem to work just fine (and I didn't see any reason why they wouldn't). To get your rig rock solid, I would think you would need at least one across the width at the back and have the wheels securely chocked. Longer rigs may need one up front. At $50 each (btw, Amazon has a similar one made by Valterra that sells for the same price with free shipping), I doubt you could make one cheaper.
The upsides I could see are simplicity, no reduced ground clearance that attached stabilizers would have, and low price.
The downsides are they have to be set up and removed everytime you move, you have to bend over or get down on the ground to set them up (fun if it's muddy), the strap will eventually fail from weather exposure (at least it can be replaced), and the ratchet mechanism on the strap could jam from dirt or mud (again, it is replaceable).
If you decide on it, I would suggest bonding a thick layer of rubber on the bottom of the pads so the metal isn't in contact with the ground (the galvanizing will protect only for so long when in frequent to constant contact with moisture). Use a mastic (liquid nails comes to mind) to ensure no water can get between the pad and the rubber. One source of the rubber could be pieces of tire tread found on the highway.