I can't tell you how many math problems I was inspired to work through for the shear joy of getting to write out my work with these pencils. I kind of suspect the original candy-color iMacs may have been inspired by this design. When I wasn't working on problems, I was using it to draw, or just pretending it was a jet (see how the clicker is just about the right position for a jet canopy?). I even cut out and bent up the plastic from a toy's retail blister pack to make wings. And you can see a hint of how it works without taking it apart.
Truly a classic design, in all the colors. Just look at eBay to see how much they're trading for: above inflation-adjusted retail. That's signal people. If you're in product development at Pentel, please, please, bring this back. And the Pan-Am, the PS513, and the PS535 while you're at it.