I got my HP-15C in high school around 1987. Two weeks ago, I changed batteries on it, and I'm really not sure I have done that more than once before.

The device is a delight to hold and use (the buttons are unmatched in their feel), and it's fun to program with instructions listed as coordinates to the buttons on the device. RPN is such an elegant and compact notation in use.

I also have a HP-48SX, but it sees much less use – the buttons are not nearly as pleasant and the screen is harder to read. For a CS crowd, the HP-16C is the one to get (I found an emulated version very handy, when I taught Computer Architecture, and while I don't really need it, I certainly would like the Microswiss version).