We do a ton of Excel import/export in our web app. The day we moved to XLSX, every one of our users cheered because the reliability of our Excel related feature

We do a ton of Excel import/export in our web app. The day we moved to XLSX, every one of our users cheered because the reliability of our Excel related features improved dramatically. The problem with XLS is that its internal format (called BIFF8) has no official specification. It has been reverse engineered to a great degree, but every implementation I've used outside of actual "excel.exe" has show stopping bugs that you will encounter at some point. XLSX, on the other hand, has an actual published spec (OOXML). There is plenty of political strife surrounding OOXML, but at least we have a spec we can develop against. It has also been my experience that "simple" XLSX files can be constructed more reliably than their BIFF8 counterparts. Because XLSX files are XML internally, there are a great number of libraries that can be used to construct the required XML structures, while avoiding edge-case errors in composition that are inherent to reverse-engineered binary formats. At a bare minimum, software authors can use something like libxml to construct valid XML, rather than some ad hoc BIFF8 serializer. FWIW, we use the axlsx gem for our Ruby app, and it hasn't let us down yet. It even supports some pretty eccentric Excel features like data validations.