POI is good, no doubt about it. Extentech has a solid product and should not be overlooked especially if you have a budget and want to have more robust handling of existing files.
The older versions of ExtenXLS had scalability problems, however the new version of ExtenXLS (4.1) has fixed this, and is now far more compatible with complex Excel files than POI -- especially files with embedded OLE objects, etc.
ExtenXLS now also not only allows you to create formulas from text strings (like: =sum(a1+b1)) but it will *execute* the formulas in memory. It can then use an XSLT to transform the Java Excel WorkBook to HTML (out of the box) PDF or whatever. A great way to re-use existing Excel files in your applications.
If you want a great supported product, at a reasonable cost considering it includes unlimited support and upgrades, check it out at