That is Sungro Sunshine mix #1 I believe. Raybo uses/used Sunshine mix #4 in his Earthtainers.
Promix as a brand name is by Premier.
Haven't personally used it, but if it is the Sungro brand, I can't recall anyone saying anything negative about it.
If it is #1, it has a starter fertilizer charge.
This the stuff?
http://www.planetnatural.com/product/sunshine-mix-1/