Yesterday, I looked at QuickBooks 2009 for Mac and while the new version is an improvement in many areas, it does not replace the Windows version in all situations. In fact, there are a number of good reasons to continue to use the Windows version of QuickBooks despite your desire to switch completely over to the Mac. Here are the major reasons to stick with QuickBooks for Windows.
If you have multiple people in your company that need access to QuickBooks (maybe AR, AP, controller, CFO or CEO) then you have to use the Windows version. The Mac version simply does not support multiple users. Period. You could also consider using the online version of QuickBooks, as long as you can live with the limited feature set.
QuickBooks Premier is really five industry-specific flavors of QuickBooks: Contractor, Manufacturing & Wholesale, Nonprofit, Professional Services, and Retail. If you need the extra features to support these industries, then you will only find them on Windows. Some of the features are pretty compelling, so I would recommend you take a hard, long look at the product description before you pass on Premier.