I find these calculators fascinating. I'm not a computer scientist, but I find it really intriguing that their processors naturally handled binary encoded decimal, and also their algorithms for calculating trig and log functions on such limited processing power. I understand the early HPs excelled themselves on returning accurate results, often more accurate than more modern machines.

Day to day I use free42/plus42 on my phone for working out quick calculations. I find the RPN works best when you're trying to solve numerical questions as you go. It also has great time/date functions which help me to plan driving trips etc, and it is programmable, so I have a few useful calculators on it for EV charging etc.