I was next to one of these today. The truck has a large, boom mounted, hydraulically driven, multi toothed roller that drives up to your loaded roll-off dumpster and beats the crap out of the trash to compact it. They are franchising it across the US and I guess one is in Cleveland now.
You save money because each dumpster is a fixed price to deliver and haul off no matter what it weighs. Apparently the cost of using Smash My Trash is low enough so you have a savings. From what I can find it costs $300 to smash your trash vs $450 to haul it off so the savings are going to depend heavily on how much it really compacts your trash.
When we used roll-offs we always had a guy carefully stacking waste lumber, etc to maximize the dumpster fill. I yelled at anyone dumb enough to place lumber crossways and create a void. The videeo of the Smash my Trash shows a bunch of pallets tossed in willy-nilly - far and away the worst case.