I was an experienced Windows & Unix programmer. It was tricky to learn the idioms of Objective-C, especially KVC and the memory management / reference counting rules. No harder than COM, but different... easier to use once you get used to it. Also it was very confusing to learn how to hook up objects to UI with Interface Builder, that was pretty confusing compared to Windows Forms which is more automatic. The Cocoa frameworks are not too hard after you learn those basics.