I still have an HP-15C I bought new in 1987. I don't use its
programmable features anymore, but just as a desktop calculator
(after getting used to RPN, I could never get the hang of 'normal'
algebraic entry again). These are probably the slide rule of my
generation - every computer science and engineering student had one.

The good thing about these models is that they last forever
(clearly), and the keys have this great, solid feel to them when
pressed. I still have some of the programs I wrote in the back of
the manual, written as a sequence of keypresses. At the time I was a
sophomore in a computer science curriculum at UMASS, Amherst, so the
programs were probably due to my discrete math class, although I
have no memory of writing them now 30 years later.

Here is one program I wrote to calculate i x j mod n:

f LBL A
x
RCL 8
/
f FRAC
RCL 8
x
R/S

to use:

n STO 8
i ENTER
j
f A

and here to calculate the base-2 log of a number, using the fact
that the base-2 log of a number, n is the same as (ln n / ln 2):

f LBL C
g LN
2
g LN
/
R/S

to use:

n
f C

Here are the specs from the manual intro:

    "448 bytes of program memory (one or two bytes per instruction)
    and sophisticated programming capability, including conditional
    and unconditional branching, subroutines, flags and editing.

    Four advanced mathematics capabilities, complex number
    calculations, matrix calculations, solving for roots, and
    numerical integration.

    Direct and indirect storage in up to 67 registers.

    Long-life batteries."

I can attest to the long-life batteries. I replace them perhaps once
every 10 years.