When I started studying electronics in 1975 I didn't have a calculator, they were very expensive. Then rumours started about an upcoming low-cost Texas Instruments calculator, and IIRC early 1976 I could buy one - the TI-30. As did many of my classmates. The next year or two I used that one, and other similar calculators which entered the marked.

Then one day a guy some two classes above me handed me a HP calculator to try.. and the RPN immediately clicked with me, I could just enter arbitrary long calculations without ever messing up anything or having to keep track of parentheses in my mind. From then on I never looked back and I was on HP calculators ever since (up to and including the Free42 on my phone today).

I have an original HP-16C as well, I used it a lot back in the day, until calculations and transformations of hex, octal and binary was so ingrained in my mind that I didn't really need it much. It's in a drawer, but it's still good. I think I'll make some more use of it now that I'm near retirement age and just doing retrocomputing.

I've heard about the supposed loss of quality of later "HP" calculators, and I may not want to buy this one anyway (as I have an original), but I'm also waiting for someone to review the keys. The keys! That's HP calculators as I learned to know them.