And to think that the Companion Cube came about as a part of game design! They wanted to make sure the player would carry it throughout the level and not just leave it behind, so they made GLaDOS talk about it a whole bunch and they put a heart decal on the sides of the box. I find that to be pretty funny. Really, I think more people should listen to the developer commentary of that game.
Though, personally, I just left the Companion Cube on the button in the dark room after activating the first lift - I didn't use it to activate the other two.