Promix works for me because
1- Relatively Cheap
2- It's light and airy
3- Let's me tailor the nute schedule
4- Don't need to go to the hydro store for it
5- Lets me go soiless, keeping the bugs away from my grow
Things I have issues with
1- The lime charge in the mix disappears at some point, requiring amending (I put in lime at the start, but am still working out the best amount)
2- In soiless, it's a hybrid of hydroponic and soil-- not a lot of agreement on Ph, cal-mag, etc., though I am getting good results so far
I think it is a total winner for blending in an organic soil mix. I am still waiting for my grow to finish out before I decide on it as a soiless medium. If I go to something else, it will be coco.