Over the past few months I've been doing a lot of playing around with development on the Mac using Cocoa. Although I do disagree with some of the design decisions that have been made, but for the most part I really like both Objective-C and Cocoa. The developer community is extremely vibrant and very helpful, a feeling I haven't had elsewhere.
One of the things I really like about Cocoa is that it handles a lot of the mundane tasks for you. There's a technology called CoreData that handles object graph persistence. You simply setup your data model and the framework takes care of the rest. This is really great since one initial hurdle I always run into when coding, and where I usually lose steam is in persisting and reading data that my application is working with, CoreData takes care of this beautifully.
Another great technology is Bindings which is built on top of Key-Value-Coding and Key-Value-Observing. In essence, Bindings allows you to visually tie the value of a control to a piece of data in your data model. This removes the need for reams of "glue" code that you'd otherwise have to write to keep your View and Model consistent. With these two technologies all you really have to do is write your Controllers.
Granted, the Controller is generally where the rubber meets the road, but since the frameworks take care of the Model and View you're free to think almost exclusively on the problems and structure of the Controller.
So far I've been really happy with the results that I've been able to achieve with only a small time investment. Between the results and how supportive the developer community is I think the Mac is a great platform to develop for and look forward to doing so in the future.
