We've been working two Earthboxes this year--our first time. Indeed the results have been amazing.
We have a mix of hot peppers in one, a sweet red corn in the other. Banana peppers are producing weekly harvests, the others are fine, but so far there's no obvious yield. The corn looks great! It's now about five feet tall, a couple of fingers thick.
The only problems we had--a couple of weeks ago (before the tornado) all the corn flopped--just keeled over and looked sad. We made sure we watered it well, propped it up and it seems to be doing ok now--except for one stalk that looks like it snapped.
The peppers were devastated by the strong winds--they flopped, got twisty and looked very unhappy. Again a bit of support, water and warm sun brought them back within a week or so.
The theory is that the fertilizer and light soil provided with the system encourages rapid growth while not providing a strong (dense) enough support system for the plants. As I said, generally things look great, but the yield is going to be the true judge.
It seems that you could build a raised bed, set up the watering system and use the dame type of soil/fertilizer mix for a bit less cash layout than these manufactured systems cost you.