Yes, it's good stuff......BUT it can be pricey and there are things just as effective for much less $$$$$$. I use Z7 by Flying Skull and although it's 2 parts, it's quite cheap and last a long time as you only use 5ml per 10 gallons of solution. I believe Hygrozyme application rates start at around 2.5ml per gallon, and then rise to 5ml and above. And, yes it's mixed in after you nutes are added and it's pH'd, then add the enzyme.
I believe you should be using enzymes with every watering. I've read a few detailed reports about how the final product weight increased about 15%, plants were taller and wider, roots systems clean and white, etc. Stop using about 2 weeks before harvest.
Bottom line, they enable the plants to do a better job of absorbing nutrients thus the plants are getting more nourishment and will be healthier.