Now with completely unscientific microbenchmarks.

user=> (use 'jsontest.core)
user=> (testit)            
Clojure version:  1.2.0
Num roundtrips:   100000

Trail:  1
clj-json                               2.15
clj-json-smile                         3.88
clj-json w/ keywords                   2.66
clj-serializer                         2.41
clojure printer/reader                 6.95
clojure printer/reader w/ print-dup    12.73
org.danlarkin.json                     21.35
clojure.contrib.json                   13.01

Trail:  2
clj-json                               1.30
clj-json-smile                         2.57
clj-json w/ keywords                   2.36
clj-serializer                         1.69
clojure printer/reader                 5.98
clojure printer/reader w/ print-dup    12.42
org.danlarkin.json                     21.05
clojure.contrib.json                   12.51

Size comparison:
clj-json        223
clj-json-smile  178