The cell contains the following data in OOXML:

<f>SUM(1,2,3)</f> - The formula
<v>6</v> - the calculated value

The formula value is imported, but the calculated value is skipped. The feature will be useful if Manual calculation option is supported: https://feedback.telerik.com/Project/184/Feedback/Details/191275-spreadsheet-add-support-for-different-calculation-options-automatic-or-manual

In some cases when cells are merged the cell borders are not correctly imported.

Provide ability for working with comments in a spreadsheet document.

When the Workbook has more than one sheet a localSheetId should be exported: 

<definedName name="xlnm.Print_Titles" localSheetId="0">Sheet1!$A$1</definedName>

localSheetId (from the OOXML specification): Specifies a boolean value that indicates whether the defined name is related to an external function, command, or other executable code.

When the spreadsheet contains empty rows and the format of some cells is set the rows are still exported to pdf or printed.  

To workaround this set the print area with the following code:

IPropertyDefinition[] propertyDefinitionsAffectiingPringing = new IPropertyDefinition[] { CellPropertyDefinitions.ValueProperty };
CellRange usedCellRange = workbook.Worksheets[0].GetUsedCellRange(propertyDefinitionsAffectiingPringing);

workbook.Worksheets[0].WorksheetPageSetup.PrintArea.SetPrintArea(usedCellRange);

If a worksheet spans more than one page, you can print row and column headings or labels (also called print titles) on every page to ensure that the data is properly labeled.

When a workbook (xlsx) contains a worksheet where some of the columns have width, bigger than the value written in the SpreadsheetDefaultValues (2000) an exception during the import is thrown.