that it will open them... but only 1 sheet;
If you try to open an excel file that has mor ethan one worksheet, you will be asked which sheet you want to open.
Obviously if you have a workbook that has multiple sheets, and the sheets relate to one another, (which is likely), you are likely to have a problem.
Depending on how complicated the books are you may have good success opening them, and then saving them again using the spreadsheet in OpenOffice, which does a very fine job of converting sheets, and is free!
Note that any macros are not supported.
hope this helps,