RPN Scientific Calculator opens quickly which is nice when I want to do a quick calculation. This version is not programmable, but if I want to write a program, I'll use something like c, FORTRAN, MatLab, Mathematica or LabView. The conversions are easy to use. What is especially important is that it has stack manipulation commands. The one that comes with the Mac does not. Also, the Mac calculator gives incorrect answers in RPN mode. Also, this calculator looks nice.