While being bound to Excel 11 (aka 2003) 10 (aka 2002)^*) and above might be bearable — depending on your environment and what you need it for —, the more severe problem I found with this approach is the internal optimization Excel performs with empty cells.

Excel will drop <Cell> tags if no "significant" data or format information has to be saved there. Instead the next cell in the output will get an attribute to indicate its column position in the worksheet instead.

Suppose you have a row which looks similar to

data

more data

…

Then your XMLSS output might look somewhat like:

<Row>
  <Cell>
    <Data ss:Type="String">data</Data>
  </Cell>
  <Cell ss:Index="3">
    <Data ss:Type="String">more data</Data>
  </Cell>
  ...
[download]

which makes it harder than necessary for my taste to navigate to the cells in search for.

Update: ^*) I now think XML-Spreadsheet (XMLSS) export was introduced with Excel 10 ->MSDN