1) OOo is the second best program to load the proprietary xls format.
2) OOo loads Microsoft's new shit since version 3. If the file in question works better in OOXML, open the xlsx and then save as xls in OOo. The file should be slightly different, but still compatible with other applications. Others reported doc and xls documents that could be loaded into OOo, but stopped working with MS Office. Things like that happen with proprietary file formats and there is nothing to do about it.
3) You can run Windows XP with MS Office on Mac PCs.
just to mention one of several web-tools. A list of ODF-capable applications
and a plug-in to make MS Office comply to the standards
. Native ODF support is announced for the next service pack of MSO2007.
5) The ODF file format has been invented to stop all format wars in the office sector. MS struggled several years to establish their so called "Office Open XML" (notice the similarity in names!) as it's proprietary counter strike, since the binding between file format and application is what they earn their living with. See the linked article in my previous answer. Writing OOXML is stupid because it increases the count of OOXML documents, fueling Microsoft's format war which they have lost already.
ODF can be implemented without any legal issues and without too many technical issues. It's a well documented, published ISO-standard since 2005, free by any means of the word. If Sun or OOo dies, the format will remain and as long as zip-compression and ASCII-encoding are well known heritage, future generations will be able to read ODF documents.