The HP-15C is the only calculator that ever became an extension of my mind. It is that good. After 20 years the keys became a little unreliable, so the brain connection snapped; I don't know if I could wire it back in again.
1975 in BASIC on Dartmouth College's time sharing system. I was 13 at the time and the local schools had installed a handful of mechanical teletypes (chukka chukka chukka chukka ding chukka...). It was a beautiful thing!
This brings back many happy memories of crawling through filthy ductwork in search of hidden spaces. I still have a box with rolls and rolls of B/W negatives that I shot in '83 to document some of the secret places at MIT. All I need is about 40 hours of free time and I'll scan them all...