I just started using the new POI 3.6 http://poi.apache.org XSSF which was released in December 2009. The updated API provides support for reading and writing in the OOXML spreadsheet format (.xslx) with Java. So far I've been pretty impressed with the API and haven't really encountered any issues with it.
What issues or gotchas have you encountered working with the 3.6 API if any?