>I'm seeing a number of Open Source libraries on CodePlex (e.g. NPOI, EPPlus, ClosedXML, etc.)
>
>My requirements are that I need to read and write Excel spreadsheets and they might be in the old XLS format as well as the newer XLSX format.
>
>I see that NPOI says it supports XLS, XLSX, and DOCX where as the others don't support XLS.
>
>Suggestions?

I've used NPOI and it has worked. I can't compare it to the others, but if I needed to do Excel work in the future I would start with that library.