I am a firm believer in it. Especially in nanos. Have used it off and on over the last 10+ years and can always see a difference in diatoms and undesirable algae growth. A good test is to run a skimmer without it and without making any changes in tank livestock additions and feeding add the chemi-pur and you will see the skimmer produce less. I keep it in return sump filter sock, but would probably work best in a phos reactor or no-bypass filter.
But on the other hand I would not recommend it if you don't do water changes often or use trace elements. It probably strips those along with excess organics like carbons do.