I believe that I have owned a total of 4 HP calculators, but the one that I like the best is the HP15C. Earlier calculators had LED displays which used a lot of energy, requiring a large rechargeable battery pack. The 15C has an LCD display, and is powered for years by 3 tiny watch cells.
At the top in this picture is my old 15C, acquired about 1983. About 10 years ago the display suddenly developed some dark patches at the edges, but this has not yet affected calculator function. Below is the new “limited edition” 15C, temporarily available. I couldn’t resist, given the prospect of my old calculator some day failing.
I also have a later model calculator, the HP48. It has a 4 line display and more fuctionality than the 15C, but it is larger and heavier, and I do not really use the extra capability. Also, it operates slightly differently in a few cases. For example, when I want to square 7 on the 15C, the keystrokes are: 7, enter, X. The 48 takes one additional keystroke: 7, enter, enter, X. I think that this change is the consequence of having a 4 line display. But I find the difference to be annoying.
North Branch Reels 
We used to call our HPs “Polish Inverse”
Nick,
I took a course in Numerical Analysis about 1970. One article in the textbook explained “Reverse Polish Notation” and assigned the problem of writing a FORTRAN program to process RPN expressions and print the result. I remember thinking, “This is pretty goofy”. Later, when electronic calculators became available, I better understood the power of RPN.
Dave
I just saw this post. I remember this exect HP from when I was a kid! My father, whose original career was engineering and later school teaching, had one of these. Let’s see, 1983…I would’ve been 6. I can remember him teaching me the different input method on the 15c and explaining how it was like learning a different language. I know he loved that calculator. Very cool post, brought back some memories.