Since the Excel XML format is a well-behaved, full featured (and thoroughly documented) format that Excel can both read and write, I strongly suggest that you use this approach.   If you are generating “stock” outputs (e.g. reports) in Excel format, templates are a reliable way to do it.   (I did this recently for Excel 2003 and impressed a lot of my client’s folks.)   If you need to do more advanced things, any XML-oriented package would be suitable and efficient.

It seemed to me that MS has done their homework on this one.   It’s a good, clean design that can handle formulas, formatting, you name it.   And I think that there’s a formal DTD for it, as well.

The final result is “exactly as the user expects.”   They do not have to do anything special, afaik, to handle the file.   Excel sees what the file contains and, “it just works.™”   In fact, I believe that their new .xslx format is XML-based behind the scenes.