I had the opportunity to use RestKit recently, and wanted to quickly post my thoughts.
RestKit is a handy framework to easily convert JSON or XML data obtained from a REST endpoint (the ease of use can be seen in this tutorial. In addition to converting a JSON response into actual objects, the framework offers caching via CoreData. Really great stuff. There's one catch though: RestKit relies on old version of AFNetworking, version 1.3.3 or so. As a result, using RestKit prevents an app from taking advantage of AFNetworking 2.0. I think for many this would flat out be a dealbreaker. While the developers are supposedly working on this, I haven't really heard anything regarding their current progress, so I wouldn't bet on seeing it emerge any time soon.
In summary, RestKit is a great means of transforming a REST response into actual objects you can use in code; but if you plan on using AFNetworking 2.0 (and if you're considering RestKit, you probably should), the dev lag makes this a non-starter.