SPB-Cooperator wrote: ... the sooner 100% comparability becomes international law, the better.

There will never be an 'international law' but there is already an international standard:

ISO/IEC 26300-1

Both Microsoft and Google Docs claim some compliance, but whether that's limited to importing ODF spreadsheets I don't know. If you save a spreadsheet in a proprietary format like xls then don't expect it to be portable.

If you're forced to frequently process data from spreadsheets in different formats then the best way may be to export it as CSV files and import them into your spreadsheet. A lot of that can be automated - in Excel at least.