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.

We have actually been using elasticsearch as our main data storage at backstitch for almost a year now. How I usually explain its awesome power is (1) take the schemaless nature of MongoDB, (2) add the indexing power of Lucene, (3) and give it the flexible scaling of Riak.

The documentation is also fantastic and the plugin availability pretty good.

It's what Solr would look like if you designed it today.

The clustering Just Works, with autodiscovery via unicast, multicast, Azure API, EC2 API, or Google Compute Engine API.

I am just starting to learn elasticsearch so am not a good source. But I didn't want something that was technically revolutionary. I wanted a tool that would be easy to get started with and provide something that is completely trivial as far as the end user is concerned (search).

It's an "it just works" technology that also goes very deep and solves a number of problems where the competing solutions are an order of magnitude more complex and/or expensive.

Something like Google Alerts could be designed easily with elasticsearch. There's probably no other technology (off the shelf) that could be used for an use case like that.