I m using it since earlier versions for almost 3 yers, i guess. It makes config easier, offers a well designed, easy API. Especially for Rails, PHP, Django crowd, an easy choice to create a sensible text search. The main benefit was an easier Lucene with JSON/http API. Yes a lot more advanced features are there too.
Advantageous may be the fact that you can extend ElasticSearch with JavaScript, whereas I think as easy for someone who understands the Solr codebase, I've wanted to try that once, but I had to resort to doing the necessary computation in the language calling the Solr service.
Honestly I'm not sure, I haven't used Solr. I was just comparing it to the relational databases I've used, key-value stores like Cassandra, and other documents stores like Mongo. It definitely doesn't replace relational stores, but IMO blows all of the other NoSQL data stores out of the water. The Lucene indexing behind the scenes just really delivers some impressive functionality.
Elasticsearch isn't really schemaless, but you can have it guess and extend the schema as fields are encountered (or you can have it treat specific field as blobs of unindexed json).