I've been using MongoDB for about 5 months now and can say that it is an excellent tool. I've been tinkering with building a very fast search engine for my latest app by combining MongoDB with ElasticSearch and it's been a wonderfully pain-free experience. While I certainly believe in picking the right tool for the job, I don't see myself going back to SQL anytime soon.
It's really straight-forward for simple usage. There's no official river for mongoDB, so for now, since I've got very small documents, I just save in MongoDB, and then index an even smaller searchable document in ElasticSearch. My front-end talks to ElasticSearch through a simple node/expressjs route, which was made far simpler by this NPM module -->
I apologize for using the term "combining" as it's slightly misleading. I actually use ElasticSearch for search functionality only. I use MongoDB for the standard DB stuff (read, update, etc...)
I'd like to hear more about your usage of MongoDB and ElasticSearch. I am building something where it makes sense to use both on the Play! framework, which has modules for both.
