(source code available on GitHub
What makes SheetJS even more impressive is the ability to query spreadsheets in-place with an SQLite-compatible SQL language called WebSQL. Check out the example on the website to see how awesome this is.
Together with a grid component such as Handsontable (source code) SheetJS allows for on-the-fly display, querying and manipulation of Excel spreadsheets.
For a more complex real life use case see this blog post by Patrick McKenzie: Design and Implementation of CSV/Excel Upload for SaaS