I love its useability and its foreward thinking approach. It really has a function for almost everything I can think of.

Ease of use and integration throughout the entire microsoft suite

The features I use are advanced formula tracing, linking between Excel and Word/PPT, formatting, and formula manipulation such as error wrap. All of them generally work very well and have become a part of daily life pretty quickly as they save lots of time. The biggest advantage to using Macabacus over writing your own macros is the ability to undo most of Macabacus' changes whereas standard macros will cancel out the undo feature in Excel.