I bought one of the Canon calculator mouses, and it works as advertised. Great concept, can do calculations, or act as a number pad for your computer, and does mousey functions well.
However, I found the buttons a little small for my stubby fingers, so I gave it to my brother, who used it for about 4 months. It sits in a drawer now.
I now use the Canon DK1000i II USB calculator with scroll wheel ($16 on Amazon) but only as a calculator.
Basically, you can get a mouse that can also function as a calculator sometimes, or you can get a calculator that can also function as a trackball sometimes.
If using with a laptop that doesn't have a numpad, or if you are short on desk space, they are worthwhile. But if you aren't short on room, there is little point to either one.