I've been playing with ElasticSearch + Tire over the span of the last week. It's a joy to use. Sunspot + Solr isn't a bad alternative, though.
Tire's docs are a bit lacking but it maps more-or-less 1:1 with ElasticSearch, so it's not too bad.
Very pleased with the performance ElasticSearch provides. The installation was a bit foreign for me personally (a java service container? openjdk 6 or 7?) but it's lightning quick and very flexible.
I chose to go the ElasticSearch route mainly due to my need for Geospatial indexing. Solr does it too, but Foursquare uses ElasticSearch and so that got me interested in learning more. Geo queries are really fast. My last experience with Geo involved GeoDjango and all kinds of obnoxious hacks to PostgreSQL to make it work. With ElasticSearch you tell it to index a point and boom you're off to the races.
I used Solr for the previous generation of FreshBSD, and migrated to ElasticSearch and Tire over a year ago and haven't looked back. The docs for both ElasticSearch and Tire leave something to be desired, but it's still so much nicer to use and a lot faster out of the box, at least for my modest needs.
Tire was what convinced me to choose elasticsearch over Solr. A lot of the features described in this document do not apply to my needs, but the speed in which we were up-and-running with elasticsearch because of tire was hard to match.