Apart from the fact that it was sloooow, impossible to repair as it was glued together, the display was abysmal and you needed to drag the manuals around to be productive.
Not a big fan of HP nostalgia to be honest, despite rather liking RPL.
I dunno about slow. Once I learned its quirks, I became one with the stack, blasting in complex equations in RPN while my peers fiddled with brackets and mode keys.
Plus I'll admit to feeling no small amount of nerdy badassery at typing in what looked like alien gibberish to those around me while they slowly WYSIWYGed their way to an answer.
The UI is incredibly slow to draw. Many users may not have realized that keystrokes were buffered (up to something like 32 key presses) so if you knew your way around the menus and dialogs you could keep typing and the UI would eventually catch up. Unlike when a desktop computer's GUI lags, the 48 series would always behave as though the GUI had drawn instantly, so you could direct input to a dialog box or submenu that wasn't yet visible on screen.
Not a big fan of HP nostalgia to be honest, despite rather liking RPL.