Ascent is the best for the Mac. It's still lacking a few features (notably importation of weather), but it's much better than GC and is very solid and reliable.
Have you looked at the beta version of Training Center for the Mac 3? It is coming up slowly feature-wise to what Ascent offers. Comparison of Activities and the ability to export individual or groups of activities and courses is also new.
I use courses a lot and with the ability to add course points and export an individual course for the 705 is a big win for me.
Another very good alternative to Garmin Training Center is MyTourBook (http://mytourbook.sourceforge.net/mytourbook/). It's open-source and written in Java so it's supported by Windows, Linux, and Mac OS. Besides having really good data analysis features and the type of GIS integration that we're coming to expect from this genre of software, this program surpasses the alternatives (at least, the ones I've tried) with its ability to import and export data. The import function was particularly important to me because I had collected about 200 records of training over the past 18 months or so in an OpenOffice spreadsheet and I did not find the thought of manually entering that data into a new database particularly inviting.
