I do NOT recommend OpenXLS. I used it for a little while but became very frustrated by the limits of the free version. If you want to write formulae, for example, you can't. Unless you buy the commercial version that the company behind OpenXLS sells, anyway.
I've been using JExcelAPI for the last few months and am quite impressed with it. It's very easy to use it to read and write Excel spreadsheets. The documentation is fairly good but it could be better. If you haven't already made your choice, I would recommend JExcelAPI to you.
I'm afraid that I haven't tried POI so I can't comment on it.
