We are planning to store millions of documents in MongoDB and full text search is very much required. I read Elasticsearch and Solr are the best available solutions for full
There are some search capabilities in MongoDB but it is not as feature-rich as search engines.
http://www.mongodb.org/display/DOCS/Full+Text+Search+in+Mongo
We use Mongo with Solr to make content searchable. We prefer Solr because
Since we didn't work with ES i could not say much about it. You can found some discussions about Solr vs ES on the links below.