I have never tried their mix. The boxes were expensive enough! I just use regular potting soil, usually Miracle Gro, but have been disappointed with it this year. It was full of sticks. Still having good fortune with it, though.
One thing you do need to use is the covers for the boxes. They really do make a difference. I had to order some more this year, and with shipping costs, they were fairly expensive (to me), but necessary.
This year I have also had great luck with some green beans I planted in one as an experiment, having never grown any of those before. I only planted 3 plants, and have already gotten 2 quarts of beans from those three.
The rest of the things I am growing are in containers, and they are doing well, but don’t compare with what’s in the Earth Boxes.
Oops.
I'm the only one in my household who eats green beans, but I had a craving for some. Now I have 12 bean plants, with most starting to flower.....I'm going to have to learn how to freeze them, aren't I?