First, it is unmaintained. The latest version available was posted on August 24, 2007. When you encounter one of its myriad bugs, you'll either have to patch it yourself or pick another JSON library. Just skip the intermediate step and use another library to begin with.
Second, it is buggy. In some cases, parsing text it just generated will return a different value from what you passed in! It's almost entirely ignorant of Unicode, and what little it tries to parse it gets wrong.
Third, it's exceedingly non-compliant. The text it parses and generates bears only a passing resemblance to JSON. There are varying degrees of conformance to the spec between libraries, based on personal preference of the authors -- I prefer strict conformance, others less strict -- but cjson is so different as to be simply unusable.
Yes, it's fast. I know. I wrote jsonlib partly because I was unsatisfied with simplejson's performance, and one goal (never truly achieved) was always to surpass cjson. However, speed isn't everything. As the saying goes, "if I want my math performed fast and wrong I'll ask my cat".
In my opinion, the only Python JSON libraries worth considering are:
* simplejson -- it's in the standard library, and should therefore be considered first and most thoroughly.
* jsonlib -- it's fast, well-tested, and standards-compliant.
* demjson -- has several options for reliable parsing of invalid input.
Last time I checked, jsonlib and simplejson's C extensions are neck-and-neck performance-wise. In some quick, unscientific tests, jsonlib reads faster and simplejson writes faster. However, simplejson's extensions are only used for certain subsets of input -- if you want to use an uncommon feature, performance will degrade. jsonlib has an implementation in pure C, which avoids this problem at the cost of complexity.
Apologies for the brain-dump, but even if you skip right over it, please remember: don't use cjson.