You can download your Spreadsheet.com workbooks as .XLSX files.
Spreadsheet.com works with other spreadsheet software. Not only can you import existing spreadsheets into Spreadsheet.com, but you can export out of Spreadsheet.com as well. Although the feature is called "Export to Excel", most other spreadsheet applications are capable of importing .xls file types, allowing you to export to more than just Excel.
Once inside your workbook, select File from the top menu bar and "Export to Excel" from the dropdown.
Your spreadsheet will be downloaded to your computer as an .XLSX file. Cells with data types that have no equivalent in Excel (e.g., Attachments, Related rows, Collaborators, etc.) will be converted to the closest human-readable text equivalent value.