We launched on all 3 at the same time. Mac press releases get alot more traction than PC. Make sure to post a demo on apple.com/games too.
Web traffic Now Boarding
Sales Now Boarding
The main takeaway here is the x2 conversion rate of Mac visitors. Knowing this should make deciding whether to support it easier, especially if you have some sales data to work with and how much mac porting will cost. Our flash/AIR solution gave us Mac and Linux for free. Linux never did amazing, but we recently got posted on some Linux sites and that percentage may be increasing.
2Dboy had a % platform pie chart in their GDC presentation:
World of Goo