I've seen many applications built using Excel. All of them were clumsy, error prone, and next to impossible to keep up-to-date.
If the end user needs an application to work like Excel for some grid calculations, then give them a tool to do so, or let them use Excel for that portion.
However using Excel / VBA exclusively to develop big Enterprise worthy applications is heading down the wrong road. It might work well for a while, but it won't be long before issues expose the weak points.
Since you ended talking about reports... yes, by all means have your application export to CSV, HTML, PDF, Excel etc. That way the user that wants to use Excel to generate pretty pie charts, and reformat/search/scan/crop the data can do so with the tool they feel comfortable with.