I have a Backpack for my iMac. They are just OK. Design could be better, as it tends to slip over time and needs to be tightened periodically. You can also move the unit with your hand easily, and the thumbscrews loosen at that point. The welding on one thumbscrew broke immediately upon me setting it up. Twelve South set me another set, no problem. Customer service is good.
I got this unit as a present, so I don't care much about the deficiencies. I would not buy another Backpack, having owned the one I have now. It is overpriced for what it is and the design is not secure. Some velcro or a zip tie can just as easily attach an external hard drive to the stand. Either one would perform better. The Backpack is better suited for a DVD or Blu-Ray drive. Less risk if the thumbscrews come loose and it tumbles.