i have been using Mma since the beginning. I first used the program on a Mac IIfx. prior to that i had used Reduce (1981) and MacSyma (1982), then finally Mma in 1988. I taught the Calculus and Mathematica series on NeXt workstations at the University of New Orleans. I have reviewed books on Mathematica, beta-tested the Parallel Toolkit (what later become gridMathematica), and co-authored three books using Mathematica, as well as provide technical know how for a number of other book publications using Mathematica. I have developed numerous packages.
These days as a mathematician supporting the Navy in numerical ocean modeling, i use Mathematica to do rapid prototyping, check solutions, typesetting, and have programmed an ocean model in Mathematica. I use Mathematica to determined the initial conditions and boundary conditions for ocean modeling simulations.
Mathematica is my favorite toy to play with and my desert island app.