Update: Check out my updated benchmarks.
Yesterday I wrote a post on parsing JSON with Apple's private framework. I thought it might be worth benchmarking TouchJSON, JSON Framework, and Apple JSON (what I'm calling the private framework). My results were very interesting.
I know I posted awhile ago that TouchJSON was really fast, but I guess I got my numbers mixed up. This time I ran each test 100 times and took the average. These are the numbers from TouchJSON 1.0.6 and JSON Framework 2.2.2 running on my iPhone 3GS compiled for iPhone OS 3.1. You can get the code on GitHub and see for yourself.
In conclusion, it looks like JSON Framework is the one to use, since you probably shouldn't use Apple JSON in a shipping app. JSON Framework was always my favorite for the clean, category-style interface. I only tested Apple JSON to see how the other implementations compared to it.