Someone else mentioned JasperReports which is probably the best choice if you already have reports written or would prefer to define your reports in XML. Ruport is a pretty low level toolset, but many of our users are pretty happy with it, especially because we make integration with ActiveRecord pretty easy via acts_as_reportable.
I would not recommend using PDF::Writer directly for anything at this point. Even if you don't use any other functionality in ruport, our Formatter::PDF adds interface improvements while fixing a number of serious issues in PDF::Writer at the same time. Most of these patches came from Austin, but do not exist in the released PDF::Writer code. (Memory patches, bug fixes, etc)
It really depends on what you're trying to do and how much you want to roll up your sleeves. Searching through the Ruport archives for Rails might prove helpful, and if you have any questions, just let us know on the list. (http://list.rubyreports.org)
-greg