Do you need something beyond "highly scalable search solution"? A lot of enterprises (and I'd guess startups too!) need good search solutions and your two players in the Java space (that I know of) are Solr and Elasticsearch. We are using Solr in our product, but the more I hear about Elasticsearch the more I want to give it a serious try.

My reason for not using it originally was that we had prototyped our solution using Compass (which was the project before Elasticsearch) only to have it abandoned to work on Elasticsearch. So I was concerned about the "one dev" model and losing something that was a key component of what we needed.

Since then I've not been impressed with SolrCloud (would love to hear good experiences, but it seems their distributed model isn't right) and have been giving Elasticsearch serious thought again.