The reporting engine is designed to produce xml or html. I am not aware of the option to produce Excel directly.
For sure, depending on your browser, you can copy or save-as Excel any HTML Page.
Depending on the structure the output is sometimes very good for Excel.
The Integrity Server has an Excel engine (this wil be used when you say "Export To Excel").
This excel Engine can be used by custom programms as well, and can produce ad-hoc Excel sheets. In fact, I have created an enhancement for the Integrity Gateway to retrieve data in Excel sheets directly - utilzing the Gateway templates concept.
To summarize: If It has to be Excel we usually go with a customization, otherwise we suggest using HTML, and if well formatted it can be copied / saved as Excel easily.