I bought the upgrade version on Amazon.
If you are a mathematics or engineering student or a working engineer or mathematician who frequently does beyond-spreadsheet mathematics, you need a math package. There are several competing products: Mathcad, Maple, Mathematica. MatLab and others. Most are very expensive items; but the student versions (full or nearly full versions in many cases) are reasonably priced.
While Mathcad covers a very wide range of mathematical tools, you will want to be sure that the package you select fits your particular work environment. Students will find that their department probably has a preferred product. Working engineers and mathematicians in large companies will get the product from their employer. If you are already well acquainted with your computational needs, the websites all have ample information or trial installations permitting you to compare yout favorite tools.
Mathcad 11 installs quickly and with no problem. The package includes a 473-page reference book, which is also on the CD. Activation is required and was painless; and you are allowed installations on a home and work computer with one license. Running on Windows 2000 Professional, I got an error message or two during heavy use the first week; but nothing that could not be ignored.
Help includes lots of tutorials and spreadsheets. I bought a couple of third-party references but found the CD and the Mathcad book better and sufficient.
The interface is simple enough; but it takes a while to get used to the tools for integration, differentiation, factoring, series expansion, Fourier and Laplace transforms, etc. on what Mathcad calls the symbolics menu.
On a few occasions I was disappointed that Mathcad would not accept perfectly valid mathematical expressions but instead required me to use its more limited notation.
Mathcad permits you to work with full sets of units and conversions are quite simple except that you cannot simply create or substitute units that are anything other than a multiple of the Mathcad standard units. In Mathcad (but not real life) the US system is: feet (ft), pounds (lb), seconds (sec), coulombs (coul), Kelvin (K). Since the conversion to Fahrenheit degrees includes an offset, conversion requires the user to create some simple functions. (This was annoying at first but OK once I got used to it.)