Personally I use Mathematica as a sketchpad for playing around with ideas and getting a feel for a problem. It is great because I can type in my sloppy half thought out ideas in a fairly natural syntax and Mathematica will generally do something sensible with them. Once I've gotten a handle on what I need to do and actually have to start getting results I leave Mathematica for matlab, python or C.

That being said I've met some really great Mathematica programmers, and the stuff they can make that program do with only a few dozen lines of code is truly amazing.