The supported functions depend on the spreadsheet application rather than file format. Which exact functions does he mean? If the functions work in Calc, they should work with Excel 2003 as well. OOXML is just another way to encode the very same contents as the binary functions did.
Add-in functions impose other problems regardless of the used application since not everyone has the same set of add-ins installed.

But this is a typical issue with Microsoft users. They always assume that everybody has the latest shit from Microsoft installed.
But today Office 2007 has a comparatively small market share due to the flop of the Vista operating system and the hardware resources required by Vista and Office 2007 for almost no extra value.
If someone uses all the features of his new toy he can not expect that others can handle the result on their computers.

EDIT: Every application gives a warning that foreign file formats may strip some of the features. You get the same warning when you save a spreadsheet as plain text, which can only store the current sheet’s plain data without formulas nor formatting.
There are very few features uniquely in OpenOffice.org, but not representable in the Excel file formats, regardless of xls or xlsx.

My name is Harry Tahiliani and I’m fairly sure that saving a working xlsx as xls where you just edited some values, formulas or common things will not destroy anything.