No explanation really, but I'm still fascinated by the old RPN line of HP calculators. There's a bit of joy every time I find a use for one--which these days mostly consists of helping with my kids homework. And it's as much about the hardware as the software--a real RPN calculator beats any emulated alternative any day. Handsdown. And I guess that's why I throw an old HP-12c in with my gear each morning.
My first handheld was the HP-25c. To me that was my pda. Through the years I've managed to misplace about a half-dozen models along the way (always upgrading to the newer model).
There's a great site that still covers technical details in minutia (hp-museum) with a pretty active forum. And it's a must visit site if you are considering collecting these relics.
I now have the following tucked away (an hp 16c, 32s, 42s, 41c, and 12c).
Here's to the hope that HP would someday redo the HP-15c scientific (sign the petition here).