First of all, easy to use. I have been using ArcGIS for almost 7 years now and I can tell you that people love it's simplicity and how to build simple maps and analysis.
I have been tracking and also using QGIS for almost 3 years now and as other have pointed, QGIS is getting closer on funcionality, but still "ESRI" is printed on peoples mind.
ArcGIS has a solid documentation, large use base and support for old products that most of other companies cannot provide. They have been investing in new technologies and spread of new concepts (which may be something or not - check out GeoDesign).
QGIS is a very strong candidate for a replacement of ArcGIS. There is one specific project, for a municipality here in Brazil (a large north capital) where we were able to stick with QGIS and they seem to be very happy with it. Customizing it is another point, but it doesn't seem hard - everyone just need to "get used to it".
One point in favor of ESRIs tool is its native cartographic output which is still the best, and looks like it will be for some years.
EDIT explaining the native cartographic output comment:
Well, most of GIS softwares nowdays can export to a different format, such as PDF and SVG, to be edited outside the boundaries of GIS scope.
I've heard that many many people uses FOSS GIS software to generate the basic layouts of their maps, and then change to something like Inkscape and refine the look and feel of that map.
Natively, ArcGIS has the best cartographic control and output of all. It has extensive labelling features, symbol creation, it's has many features of a vector graphics software.
Thats what I meant as native cartographic output :D
Best Answer
I think ArcGIS for Personal (formerly called Home) Use is a great deal. I have not purchased it personally, but only because I need a commercial license for my own work. I know a number of Australians who have purchased it to upskill on ArcGIS for Desktop at home.
The "functionality as compared to the usually expensive product" (i.e. ArcGIS for Desktop) is identical, because it is exactly the same software (and at the Advanced level with many extensions), just with a different license i.e. non-commercial vs commercial.