Is the syntax weird and crazy? No doubt. It is, however, an extremely pragmatic approach. It's completely unique in that it offers a dynamic, very-high-level OO language on top of C, giving you a way to work on both levels. C (or C++) for the performance-sensitive parts and integration with C libraries, and ObjC for the glue that orchestrates the application. ObjC isn't without its faults, and if was starting to show its age, so it makes sense for Apple to develop Swift, though between the two languages, I think ObjC is the more innovative one.
It's a lot like having a scripting language around a C core, though ObjC has stronger static typing. Dynamic dispatch, late binding, and interfaces (protocols) results in a super rich, dynamic ecosystem of objects all interacting with each other, fulfilling the promise of Smalltalk better — in the sense of being the basis for real, mainstream products, including macOS itself — than any implementation of Smalltalk ever did.