If you are refering to the slow release pellets i would stay away.
last year i had two friends who used those on their plants, I had the same clones as one and a 4th friend grew the same seeds as the other heavy harvest guy.
for the clones I grew organically (bat guano, worm castings, fish emulsions), and my friend used heavy harvest. his buds were darker and harsher and not nearly as good as my organic buds.
The other two people who grew from the same seeds, one used heavy harvest and the other used something else probably just scotts shit. once again the heavy harvest buds were darker not as crystally and not that nice while my other friends buds where better a nicer colour and smell and even smoked better.
So thats what I know about heavy harvest, I myself have never used it but my close friends have, your plants may grow decent but the final product will probably be abit of a let down