Comparison of JSON Parser for Objective-C (JSON Framework, YAJL, TouchJSON, etc)
If you're looking for speed, take a look at JSONKit. It is (usually) much faster than the already mentioned JSON libraries, often an order of magnitude faster. And because of it's "recently instantiated object cache", it will (again, usually) use less memory as well.
Considering a comparison that I did, the fastest library is JSONKit, followed by NSJSONSerialization (now public API) and NextiveJSON. SBJSON and TouchJSON are a little bit slower than the previous 3 libraries.
The comparison has been done using an iPhone 4S with iOS 5.0.1.
There's also this parser:
I don't remember how I stumbled upon this one, but it looked lightweight and simple, so I thought that it suits my needs. I event forked it in order to add initialization from NSData.As I needed to parse small responses, I didn't care about the performance. It would be interesting to know how it performs though.