We can compare RestKit to some other popular or similar solutions:
AFIncrementalStore (https://github.com/AFNetworking/AFIncrementalStore): A new library from the creator of AFNetworking that works in a tight connection with Core Data. It is not yet very customizable, but still in its early development and has some possible bugs/performance issues.
MagicalRecord mappers (https://github.com/magicalpanda/MagicalRecord): It is still in development, not well-documented, and not actively used by developers.
Parse (https://www.parse.com): Can be used only with their web services.
KeyValueObjectMapping (https://github.com/dchohfi/KeyValueObjectMapping): It is a small new library that is not documented yet, not fully-tested, and possibly has some performance issues.
SLRESTfulCoreData (https://github.com/OliverLetterer/SLRESTfulCoreData): It is new and documentation is a bit confusing, and the code is not documented.
Mantle (https://github.com/github/Mantle/): An interesting library from GitHub for creating a model layer. It is designed to be used more as in-memory storage, doesn't connect easily with Core Data, and needs some experience to set up and configure every model.
As we can see here, there are numerous libraries that do mapping of objects, or simplify the development by making hidden requests while fetching data from Core Data. Some of them are quite interesting to check and study—depending on a type of project one is doing. The downside of the earlier-mentioned libraries is that these solutions are relatively new, still in active development, or quite complex to set up.