Jim Washington compares Python implementations of JSON, including my now-quite-old json.py. It's slow relative to the fastest implementation, but the intent always was to have a fairly clear implementation without trickery or dependence on other modules.
Bugwise it holds up pretty well to Jim's tests. I've not worked on it since JSON adopted scientific notation, and so all those exceptions about finding an "e" when parsing a number.
There are a few other problems, but it holds up pretty well.