I have the HP calculator on my Android phone. Just a natural to use as any HP calculator. My college graduation gift was an HP25. I wrote my first program on it to replace tables in the Schaum's Genetics book. I carried the HP16C while teaching my embedded systems (6800/8085) course from 1980 to 1983. All hand-coded assembler. Having the 2's comp arithmetic support was helpful for long relative branch back calcs. The HP12C serves as my financial tool. I did finally cough up for a top of the line HP graphing/programmable calculator. Very nice. I keep a lower end one at hand when writing the aeronautical engineering calculations. Sufficient for that kind of work.
It’s nice to know I’m not the onlt RPN geek left in the world.
By the way, I just found these guys last night:
https://www.swissmicros.com/
They make HP “clones” that actually look pretty nice. I may take a chance on buying one.
I have 20+ different HP RPN calculators and a few Soviet made RPNs as well; but the newest one is about 20 years old. I’m not sure I want to subject any of them to the rigors of a daily ride in my briefcase.