Excel Workbooks; ISExcelCell.Formula does not work

The ISExcelCell.Formula property in an Excel Workbook does not work - the formula is never set.

Note, the Cell's formula must start with an equals symbol, e.g. "=A1+A2". In addition, formula's do not calculate the resulting value. However, when you open a file in MS Excel, formula results will be calculated automatically - alternatively set the Cell value rather than its formula.