Everyone seems to have an opinion on them, some folks like them and others hold them in close proximity to the plague.

I bought a great big box of them to goof with (they make great puzzles), with the intentions of experimenting with a coil denitrator and using them there, which has yet to come to pass. I did use them in the 'crash chamber' from the overflows from our 55g to reduce the air bubbles in the sump, 10' drop into the basement, so it really brought down a lot of air. They worked for what I wanted, but they were not in play long enough to really form an opinion of their effect on nitrates.