> What the Circlean thingy does is more similar to an antivirus

This is also how I would characterize it.

> Whether this is effective against everything is another thing, of course.

The homepage states:

  In the worst case, only the CIRCLean would be compromised,
  but not the computer reading the target (trusted) USB
  key/stick.

which (at least to me) seems to imply complete effectiveness, which is what irks me.