I'm sure you've noticed that there are some bugs with ExcelPackage. Patches have been posted but not tested & integrated into the code. I'm not sure what the original developer(s) of this project intended, but there's an effort to keep the project going
If anyone knows the state of ExcelPackage vs. the OpenXML SDK, I'd love to hear your thoughts as I consider adding my efforts to extending this great project.
ExcelPackage vs. OpenXML SDKs