It manages the prebuilt binaries...but requires being run as root or adding myself to the list of Nix trustedUsers...on an OS level.

Both of those things are scary and dangerous. I don't know what your software or Cachix is doing.

Can you tell me exactly what it will be doing to my system? What changes? Does it use a lot of disk space? Is it easy to undo? And is there anyone who recommends doing it who isn't associated to IHP?