Also Good, though I wonder if either of them have been adapted for Office 2007 yet.
The only thing I didn't like about POI, is that the library is so big. JExcel is much smaller, and yet I haven't found anything (yet) that it can't do that POI can. Then again, the reverse is also true, I haven't found anything POI can't do that JExcel can.
I have to admit though, that I definately have not had extensive experience with both of them. There are, I am sure, things one could do that I haven't attempted, yet.