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
You can download Mac OS X installers from this website: http://www.kyngchaos.com/software/postgres
I don't know this is compatible with what you installed using brew, but the PostgreSQL/PostGIS binaries you can get from that website should work well, too.
Best Answer
I still have to resort to ArcGIS in a virtual machine from time to time on my Macbook, and have experimented with performance issues over the years. As @Chad Cooper mentions, XP will feel very snappy indeed ... so does Win7 but it's worth taking the time to tweak it (get rid of Aero, replace the default 'find' with 'everything' app, etc; lots of advice about that online).
As far as the windows VM is concerned, it has its own hard drive and the Mac doesn't exist - but the VM software lets you share folders which appear as network shares.
As far as performance goes, the interface with the file system is, indeed, your biggest issue. The most convenient way to work with your data is to keep it all in a shared folder within your Mac filesystem, so that files are accessible to both. That's a great way to keep track of your data, particularly when using something like GISLook to preview rasters and vector data in OSX. HOWEVER ... you take an enormous performance hit when the VM is accessing files on a shared directory!
When testing GIS operations in a not-terribly-complex project (2 DEM rasters, 20-30 simple shapefiles), both calculations and display were affected by the location of the files:
This was with VMWare 3.x 18 months ago or so - but things change, and I hear the performance of both Parallels and VMWare and Virtualbox are much improved in comparison!
My current solution: I have a "Data" D:\ drive which is a virtual hard drive (VirtualBox) that mounts with the C: system drive - I synchronise the data directories with shared folders on the mac to keep things tidy. Keeping the D drive separate keeps its size down and lets me back it up separately in Time Machine.
Tip: create a 'sparse image' in Disk Utility, and create your virtual hard drive inside it. Let Time Machine back up only that image, not the main C drive (main VM file). "Sparse disk images" are interpreted differently, and only the individual blocks of data that have changed get Time Machined on backup, instead of the whole giant file.