I have used both iText and the FOP approaches.
IText is good if you want to read data from a database and directly format it into a PDF file. There is no XML involved here. It is good for generating small PDF's on the fly for display in a browser.
In the FOP way, you have to format the data you read into XML, then create an xsl stylesheet using formatting objects, the xml is then rendered into a PDF using the xsl stylesheet. If the document is large, the transfomation is rather slow and is better done offline. However this way gives you more flexibility on the PDF. Easy to modify PDFs quickly... It will take a little longer to setup and learn if you have not worked with XML/XSL before.
