I bought one of these Faber Castell Addiators on eBay a few years ago because I was curious as to how they worked, and how useful they could be: They're very simple to use. But it's still quicker to use pencil and paper for addition and subtraction.
However, in combination with a slide rule, the addiator is useful. The results from calculations on the slide rule can be stored on the addiator for later use.
Video from rregister
Thanks Retro Thing