Though I'd add in my 2c to the debate ;) I've just started evaluating FOP for production use in our company. We have some code documentation in XML format and can use XSLT to create FO, then PDFs and/or HTML - very useful.
I had downloaded FOP 0.19, and was becoming quite frustrated with some issues. To be fair: all the basic stuff worked with no problems. But I was trying to recreate our technical documentation cover page and headers/footers, with little success. For I start I needed a page border, which I have still not discovered how to do; then I needed tables with cells spanning rows, which I found to be broken. The list of other little niggles goes on.
So I decided to get the latest CVS version, and try with that. I'm *extremely* impressed :) I haven't tried the page border again, but most of the issues that I was fighting with seem to have been resolved. FOP seems quite capable of reliably producing attractive layout, which is pretty much as much as you can demand from a program of its nature.
I think a disclaimer or warning is very prudent, albeit becoming less justified.
I'm personally very pleased that FOP gets used. I'd be less interested in
working on the thing if it wasn't. Unfortunately we have to issue some blunt
disclaimers occasionally, along the lines of DO NOT USE FOP FOR PRODUCTION;
if you know what you're doing you can interpret that how you want. :-)