I'm surprised speed benchmarks are left out. There are a number of posts that do some simple tests. I like cjson because it's so fast -- it performed about 50x faster than simplejson for the strings I was decoding.
It should be noted that simplejson is now included in the standard library. Reference: <a href="http://bugs.python.org/issue2750" rel="nofollow">http://bugs.python.org/issue2750</a>