I've been playing with ElasticSearch + Tire[0] 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[1] 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.