In our Clojure web app, we started out using MongoDB, and eventually we migrated to Datomic. It's a nice compromise between relational DBs like SQL, and "dynamic" DBs like NoSQL. So far it's been a huge performance win. Queries are faster here because we have relational data, and we don't do too many writes, so the trade-off worked perfectly for us. Plus, the API is way nicer to work with than SQL-based anything.