RPN is a solution to the ambiguity you get when you are entering a string of things that are not displayed and are calculated one by one. It makes sense for a calculator which only displays numbers but in my opinion doesn't make any sense for other devices. Rather than learning the correct sequence to enter operations, I would rather just enter a formula. E.g. not
Ok, sure, I may have to use parentheses but at least if I enter (2+5)^3 I can see exactly what I've got. There's plenty of room on the display to show the formula that was entered, and a history of recent calculations. Use the keyboard for the numbers and just have buttons on the screen for functions that are not easily accessible from the keyboard, which already has / * + - () % and !
AdvCalc looks good, though, and I'll use it till someone makes the calculator of my dreams.... The other non-RPN calculator, aside from general shininess issues, also had some non-functional buttons and too many functions hidden until you touched shift, IMHO.