I decided to run a little shootout (the link is to results and the code used to test).
In terms of speed,
clj-json is the fastest, 1.7x
cheshire, and 5.6x
clojure.data.json for a simple parse/generate task.
clojure.data.json has the smallest footprint, and
cheshire has some superb features, though, and is my preferred library for dealing with JSON. You get support for SMILE, as well as a lovely interface for interpreting JSON (adding types, special rules on keys, etc) and custom encoders (the last also found in