I saved all summer in 1973 to raise $395 for an HP35 to replace my slide rule at the beginning of the 1973 school year, but by the time I had the money to order it (from an ad on, I think, the last page of August 1973 Scientific American), HP had introduced the HP45, and so that ended up being my first electronic slide rule. I still have two of them, both work, but one has several missing LED segments, so isn't terribly useful. I use an HP45 simulator on my cell phone when I need a calculator for the occasional shop calculation (typically simple trigonometry).

I also have the slide rule that the HP45 replaced.

As long as we're reminiscing about HP - in 1976 we took delivery of HP 2640A ASCII CRT terminals. HP built a character mode terminal with a backplane that took add-in cards for storage and other purposes. They came with a program you could run an an HP 3000 minicomputer (for which the 2640 was designed) to download diagnostics to the 2640. We hacked the download, and figured out how to program the 2640 - basically turned them into an early personal computer, 5 years before the IBM PC, 2 before the Apple II, and contemporary with the early backplane personal computers like the IMSAI.

Fast forward to today, when there are entire computers, complete with batteries and Bluetooth Low Energy communications that fit in the margin of a contact lens and can be used to monitor glucose levels in tears, or focus the lens with piezoelectric components.