I personally use JasperReports as my open source reporting tool. I forget why I didn't care for JFreeReport when I tried it out (it was a while ago), but I found that I liked Jasper better. Of course, getting the iFreeReport GUI designer is quite valuable. Since report layout is a visual thing, it should be developed visually.
Piscis Babelis est parvus, flavus, et hiridicus, et est probabiliter insolitissima raritas in toto mundo.