My local stopped selling used kegs because it was becoming so difficult to get decent ones. I think their supplier gave up, as well.
Back to the original topic, though, I have not used these nor the Tap-A-Draft system. What I have found with the mini systems I've used is that they each have some sort of draw back.
I'm not sure I get the purpose of these boxes. If they're for taking beer to parties, growlers work great and pretty much every one of us has growlers lying around. If they're to be a replacement for kegs, I think there are a number of draw backs.
First, cleaning. How are you ever going to ensure that thing is clean? I've used 2.5 gallon water bottles that are a similar shape and eventually they start to grow mildew because you just can't get the inside to dry out.
Second, carbonating and/or dispensing. Reliance on the mini CO2 injector means you'll never be able to dial in your pressure. You probably would need to naturally carbonate and then use the injector just for dispensing. The injectors release a LOT of pressure quickly. I've over pressurized mini-kegs and had the bottom pop out. It's easy to do, because you have no regulator. You might could get one of those mini Leland regulators, but they are costly.
My recollection of the tap-a-draft is that you can't really force carb them, either. But I could be wrong.
If you need a small system, I'd spend a little more and get the small ball-lock kegs. They'll still be usable when you upgrade to a keezer. Or, if you give up and sell them you'll get your investment back.