Note: I am the President of ClueTrust, makers of Cartographica, so you should take whatever I say with that in mind.
There are indeed a variety of solutions of GIS available for the Mac. If you look up GIS for Macintosh, you'll probably see an article that we wrote a few years ago giving a basic rundown of the Mac options at the time, including Grass (still available), qGIS, ESRI ArcExplorer J, and others. Since that time, we've released Cartographica, a Cocoa-native Macintosh GIS designed for exploration, analysis and presentation of Geospatial data.
My experience tells me that each product has it's own niche.
Those who need compatibility with ESRI's geoprocessing languages generally tend to spend the $1500-$25,000 and load up a copy of Windows with Parallels or VMWare to get exactly what they could have in the most expensive systems on Windows.
Those who are willing to trade time and effort for money tend to take the qGIS route. It's Mac-like (being based on QT), but it is multi-platform and free.
Cartographica (you can check out the web site above if you are interested) is aimed at users who are interested in a commercially supported GIS on the Macintosh with growing analytical power and built-in features like server-based, multi-country geocoding, support for server-based and file-based GIS data, and a UI that is truly Macintosh.
Ortelius is more of a map making program than a GIS, but if you're interested in making high-quality maps using existing data sets, it's an interesting, and inexpensive solution.
MAPublisher (from Avenza) is an add-on to Illustrator that is aimed at extremely high-quality map making. The tool is not cheap, but it is highly flexible and provides a lot of automation for styles and presentation within the Illustrator environment. (Note: this is also cross-platform).
I, for one, invite people to shop around. We know there are other solutions out there (and our blog details them with links) and we offer a 30-day trial of our software, and I'm quite certain that Ortelius and MAPublisher are also available with trial periods for those interested. And, I'm quite certain that everybody offers some kind of educational discount as well.