I still use my two vintage 15Cs for simple calculations on a regular basis, and change the batteries every decade or so.
To be fair, lack of self-discharge in the battery chemistry has a lot to do with the long battery life; you can actually drain the batteries fairly quickly with long-running calculations.
I've had other HPs (28C, 48SX, 48GX, 33S, 35S, 49G, 50G, Prime) but the 15C is the one I keep returning to, as anything more powerful really should be handled on a computer CAS like Mathics or Matlab-like environment like Octave instead.
To be fair, lack of self-discharge in the battery chemistry has a lot to do with the long battery life; you can actually drain the batteries fairly quickly with long-running calculations.