I am taking a circuits class with 20 other students, all of whom are using ti-89s.
doing AC steady state circuit analysis is loads faster with the hp.
for example, find the parallel equivalent for a 20, 30, and 40 ohm resistor:
20 inv 30 inv 40 inv ++ inv
11 keypresses vs. 19
When you get to complex numbers (inductors and capacitors) it's not even close. I finish calculations in well less than half the time it takes the TI users. And not because I'm some sort of superwhiz with the calculator- it just works better.
The TI is a good calculator, but you can't really appreciate the speed of RPN until you've taken the time to get practiced with it.
The HP also has an algebraic mode, fwiw, if you want the "how it looks on paper" effect. It can be helpful if you are working with a complex equation and want to make sure you have done it correctly.
So.. RPN isn't god's gift to calculation for everything, but it can be very handy in many situations.