Here's a good video on the subject from ElasticSearch's creator: http://vimeo.com/26710663
ElasticSearch has very little ceremony around creating a new index and getting started with using it. You will eventually need to do some configuration to tune its behavior for your specific application, but the learning curve is nice and gradual. This makes ES great for exploration.
The JSON document store aspect of ElasticSearch is indeed very nice. The RESTful API is simple enough that you don't really need a client, just grab your favorite HTTP client library and start integrating. Plus, coupled with solid distribution, you're looking at a pretty viable standalone data store, IMO.
Also, very good documentation. And its user/developer community is all full of the really smart, enthusiastic early-adopter types right now :)
Not least, Lucene itself is hands down the last word when it comes to search.