One feature that I strongly miss from Objective C are namespaces. Inability to avoid name clash between your classes sometimes really stands in your way.
I am reading S. Kochan's book "Programming in Objective C" and in one of the examples he creates the class Point. Unfortunately, there already is struct Point defined in mactypes.h.
The solution is easy - rename the darn Point to something like TKPoint ... at least that is what Cocoa naming guide recommends. It is probably matter of getting used to it - but I still find the solution suggested by Kevin Hoffman much more elegant - too bad it is only theoretical. Much nicer than NSHack ;-)
I have started to seriously play with Objective C and XCode about week ago. So far it was very pleasant experience. I really like the language - feels like Ruby (or Smalltalk if you want) with rocket launcher: very powerful, blastingly fast and dangerous like hell. No array boundary checks as in Java :-).
The XCode was not bad experience either. Here my very long Eclipse and Visual Studio history is going a bit in the way, as the things often works quite different. Not necessarily worse, but different. It would be too early to judge it - not before I get more proficient in both Objective-C and XCode and learn more Cocoa goodness.
Lured by the iPhone platform, this is what I read in the evenings. Feels kind of funny: after 10+ years of Java/C# and almost as many years of C++ beforehand, I mostly lost my C instincts and C habits. Fortunately, I have also mostly lost my C++ bad habits as well.
The Apple site is definitive starting point - put the http://developer.apple.com/referencelibrary/index.html into your bookmarks. From non-Apple sources, very bookmarking-worthy is the Cocoa Literature and Cocoa Devcentral.
There are few technical books which you enjoy first time you read them, remember the experience and keep coming back. One of such classic books is The Pragmatic Programmer - from journeyman to master - by David Thomas and Andrew Hunt. If you have not read it, get it, it is worth every cent. Since the book was written - ages ago, back in 2000 - it did not loose anything of its freshness and value - which one cannot say about most of technical books written earlier than 2-3 years ago. Inspired by the success of their initial creation, Dave and Andrew started the website and continued writing whole Pragmatic Programmer series of books.