It would be beneficial for the clients if every cell has this "cached value" even if the formula can't be evaluated for some reason with the SpreadProcessing - no need for recalculation, just reading the value. It should read the last values while the document was being saved at the originator.

Getting a method like "GetCachedValueString ..." , ideal with the same formatting used for the cell and localization, would be helpful. 

Incorrect calculation of UsedCellRange when conditional formatting is applied to a large cell range.

Workaround:

var usedCellRange = workbook.ActiveWorksheet.GetUsedCellRange(
    CellPropertyDefinitions.AllPropertyDefinitions
    .Except(
        CellPropertyDefinitions.AllPropertyDefinitions.Where(p => p.Name == "DataValidationRule" || p.Name == "ConditionalFormatting")));

ArgumentException  at Telerik.Windows.Documents.FormatProviders.OpenXml.Model.Elements.Theme.SystemColorElement.GetColor(IOpenXmlImportContext context)

The problematic value is val="windowText"

When an xlsm file is imported and then exported, a new ThisWorkbook object is created in the VBA browser. This creates issues when making VBA functions that have to be executed upon opening of the file.

Applying text format to a column also sets the format to the next column.

This happens when the columns have a size different from the default and two consecutive columns with different format also have the same size. This can be worked around with a slight change in the column sizes:

int columnCount = worksheet.UsedCellRange.ColumnCount;

for(int i = 1; i < columnCount; i++)
{
    ColumnWidth colWidth = worksheet.Columns[i].GetWidth().Value;
    if(colWidth.Value != SpreadsheetDefaultValues.DefaultColumnWidth)
    {
        ColumnWidth previousColWidth = worksheet.Columns[i - 1].GetWidth().Value;

        if(colWidth.Equals(previousColWidth))
        {
            worksheet.Columns[i].SetWidth(new ColumnWidth(colWidth.Value + 1, colWidth.IsCustom));
        }
    }
}