I wonder if such a compact unit could have an active converter...
For a proven and compact converter, I'd suggest the Belkin PS/2 to USB adapter, which works with some things that the Blue Cube will not. It's one of the more expensive ones out there, but such is life.
EDIT: Whoops, didn't see your link where you said it was tested by Webwit. Still, I wonder what the problem with it was.