With any non Microsoft Office suite (iWorks, for example) there will never be 100% compatibility between them. There are functions that are proprietary to MS Office that other apps just can't duplicate.
Now, for most of the personal world, this doesn't matter. Those functions lost on a cheaper suite are not ones used by most of the people who have computers. The issue comes in with the true business power user who relys on the "unofficial industry standard" of Microsoft Office and those specialty functions that are only available to user of Office. They want, require, 100% compatibility when producing a deliverable to management, clients, vendors and what not. Again, most people (even in business) don't require the higher end functionality, but they do require the 100% compatibility.
The other main thing to look at is the price point of Office:mac 2011 Business. The full version is still cheaper than the Windows version of Microsoft Office 2010.
Personally, anyone in business who says they need to be able to read/edit MS Office docs, I will always point them to the full Microsoft Office suite rather than iWorks or OpenOffice or the other cheaper/free suite. Simply for the Compatibility. And OS doesn't matter in this regard.