I haven't used FX.php, but I've been very impressed with my own application's stability using RFM. I haven't had to restart my Filemaker server in the past six months and the only problem I've encountered were timeouts when the company hosting my server had brief routing problems. I'm using it as a read-only database and haven't yet opened up the data on the backend Filemaker server to users other than admins, so I haven't stress-tested it in any way.
I'm hosting my Ruby application on Heroku with Filemaker on my own MacMini server in a co-location facility. I use Memcache locally to cache results. I have been thinking of using CouchDB, pulling in XML from Filemaker Server and pushing JSON out. Actually, I initially brought in RFM purely as a way to get at the data, not thinking I'd keep it as a live backend. What's keeping me with Filemaker at the moment is the speed with which I can edit data on a native Mac client.
I've just done a quick successful test on JRuby with an RFM installation of "jruby-1.6.0.RC2" on OSX. To install RFM I ran "jruby -S gem install lardawge-rfm" which first installed "weakling-0.0.4-java" and "nokogiri-1.4.4.2-java" before installing "lardawge-rfm-1.4.1.2". So, it does install the Java version of Nokogiri for you. The code I wrote was very basic, opening a password-protected database, going to a layout, finding a specific record, getting the contents of two fields and displaying them concatenated.
This is a very quite mailing list, but I think that's because RFM works very well.
Stephen