What's not true about Microsoft Office files being interchangeable between Office for windows and Office for Mac ??
Nothing's not true, that's what's not true !!!!
What I believe you mean Max is that iWork can load those file types too. If this is the case, then that's fine, because when I worked daily on macs we always used Microsoft Office for Mac and older versions of Apple's built in software did not do this reliably, if at all in some cases. Office for Mac sucked under OS8 and OS9 but improved considerably under OSX.
Iwork is a very lightweight application in comparison and we never used it.
From macrumours - comparison of iwork and microsoft office http://guides.macrumors.com/Comparison_ ... oft_Office
Microsoft Office for Mac has excellent support for Microsoft Office for Windows files, though there can sometimes be minor compatibility issues, Office 2008 can import both traditional Word documents and new Office 2007 "Open XML" Office files. iWork also has support for both traditional and "Open XML" Office Files but can have moderate compatibility issues with Word files containing complex formatting or Word-specific features — as will be the case with all word processors which must reverse-engineer Microsoft's file formats or don't include the exact same feature set. Some very old Word files will not open in Pages, but they will generally open in TextEdit. If you use iWork and you want to send files to Windows users that don't require editing it is probably better to send them as a PDF.
Although iWork can open and save Office files, file management can be inconvenient. iWork automatically converts Office files to iWork formats upon opening, so when saving, users must "export" files to Office format, even if they were in Office format to begin with.