Nothing beats having a physical calculator, so don't bother with apps or desktop programs.
Well that was a bit of an old thread resurrection.
With regard to the "physical calculator", that rather depends on what you regard as 'physical' these days with smartphones etc. Personally I use Neocal on an old but perfectly serviceable Palm TX, perfectly physical to me.
Anyway, the point I want to make is that these devices can normally allow you to view the entire RPN stack, not just the X register. This is invaluable in both ease of learning and reducing errors in operation. There are very few 'physical' RPN calculators (especially the 'classics') that display more than one line.
Or, full stack view only hamper the effort to learn RPN proper. I don't know the answer, but I would guess that this kind of checking is a bad thing. Like counting parentheses (thank you bitseeker) on a multi-line algebraic calculator. You should probably commit the "RPN routine" to muscle memory and not invoke conscious thought to "confirm" everything. It's like the difference between looking at the keyboard as you write, or develop a more low level feel of what key you stroke and why. I would think that you develop a more deep, unconscious, understanding of RPN (and the same thing goes for algebraic mode as well I would think, but RPN is more suitable for hindbrain operation to off-load the frontal cortex).
A physical calculator is obviously a
calculator, not a general purpose thingy (smartphone, desktop, tablet, etc), with a proper keyboard whose layout you can commit to muscle memory and such. This applies to algebraic calculators as well, of course. I think this is an important point in the discussion and not only is RPN still relevant, but physical calculators are likewise still relevant.
(Btw, "numpad calculators" connected to a computer via USB or PS/2 port are a thing, but for some reason I have never seen a decent one. Scientific, RPN, programmable, etc. No need for leaving the current program/window on a computer, just do the calculation and press a key to insert it exactly where it is needed. *That* would be progress.)