The solution that we used, needed to:
- Allow Reading/Writing of Excel produced files
- Be Fast in performance (not like using COMs)
- Be MS Office Independent (needed to be usable without clients having MS Office installed)
- Be Free or Open Source (but actively developed)
There are several choices, but we found NPoi (.NET port of Java's long existing Poi open source project) to be the best:
http://npoi.codeplex.com/
It also allows working with .doc and .ppt file formats
