I get annoyed by the ways the OS X calculator deviates from HP RPN.
The main one is that the pi and e keys act as if they are invoking constant functions that return pi and e, respectively. On an HP calculator, the pi key enters pi.
That may sound like the same thing, but it is not. Consider this sequence:
2 ln pi x
On my HP 15C the result is to multiply the natural log of 2 and pi. Same thing with PCalc on my iPhone. With OS X calculator the result is either pi with an error beep if you started with an empty stack or pi times whatever was on the top of the stack if you started with a non-empty stack.
The "2 ln" part leaves ln(2) on the stack. On the HP and other sensible calculators the pi key pushes pi. On the MacOS calculator, it replaces the top of the stack with pi.
They also had some problems where if you did a function and then started entering digits it would overwrite the function result, but I think they fixed those. I've seen what I thought was that problem recently, but when I finished entering the number and hit enter or invoked a function on the number, the previous function result appeared second on the stack, so it apparently is just a bug in displaying the stack.
After using a 41CV for years, I bought a used 28S off someone. I just never cared for it. Probably just too accustomed to the more traditional RPN calcs.
But mostly I use my old 28S which is next to my keyboard always. Also have a 50g on the desk, but I prefer the 28S.