Ferret the ruby implementation of lucene is reasonably powerful, however online discussions in 2008 seemed to indicate ferret had many stability issues and would segfault regularly. There have been 10 or so commits this year so the project has pretty light activity.
Is Ferret stable enough to use in production?
It seems that the community has pulled back from Ferret and the two primary contenders are Sphinx and Apache Solr.
While I do not have any hard evidence of "the community pulling back" (yes, its subjective) it just seems like there is not much inertia behind it and I think there are more feature-rich and mature options (again, Sphinx and Solr to just name a few).