Ok, so I received the anchors in the mail today, and just got done using them to plant a few things. First off they are NOT buoyant as OVT stated. Alone they sink right to the bottom of my tanks. They are flexible and rubbery perhaps made from a silicone like material. I used them to plant 2 things, some anacharis and bacopa. The anacharis they pulled right to the bottom and didn't require any gravel to be placed on top of the disc. The bacopa must be more buoyant as it would float up a bit. I moved a small bit of gravel onto the disc and problem solved. They seem easy to use and so far I'm happy with them. You can easily cover the disc up so you don't see anything exposed. On the package it specifically says "non toxic" and "FDA food contact safe material". I used to run restaurants and this means the FDA has approved this stuff for any food use, i.e. you could make drinking cups or plates out of them if you wanted and would not leach any chemicals or bad stuff into food. That's good enough for me and IMO, much better than the lead weight strips sold commonly for plant anchors.
BigDog, How do your plants push them off? It seems like the roots would grow underneath and then the plant would be on the top side so seems like it wouldn't be able to free it self. Maybe I'm just confused. If they are reusable even better.