Comment by rented_mule

I can't think of another electronic device that has survived and remained useful for half the 43 years my 15C has - through two years of high school, a math degree, and an entire software engineering career. When I calculate something any other way, it involves thinking about what my thumbs automatically do on my 15C and translating that.

When I'm away from it, there's a wonderful emulation of the 15C (and 16C) at https://jrpn.jovial.com. The numeric algorithms behave identically as far as I can tell, and it even includes the back panel (3-dot menu near the logo, then "Help"). The visuals make me feel right at home. If only they could reproduce the tactile experience.