This baby, I was studying electronics and telecommunications in Liverpool UK in the late 1970s and I got myself one of these, a TI-57
I was amazed at this calculator, I'd never programmed a machine before and found it very absorbing. I later got myself this one, a PR-100
The TI was well constructed but the PR-100 was a good machine, these were truly my and many peoples' first real experience of programming.
As for microprocessors, this was the very first and I did a lot with it, even made a crude robot where each of two wheels had a motor and I used a bank of relays to stop/start/revere the motors, it was crude but a lot of fun, had two 6V lead acid batteries, this was right about the time Star Wars got released, very timely.
It was a kit, very exciting at the time, had a 1MHz 6502 with an extra INS8154 IO chip. It also supported the CUTS tape standard so I could save/load code, that worked quite reliably too, remarkable.