I have been searching for something like this that does NOT run on mac for quite a few months now. Unfortunately I think that due to the relative obscurity of the Objective-C language that nobody has ever bothered producing such a full featured IDE for it. Until now, and we only have Xcode.
Using JBuilder I fell in love with the auto-completion and displaying the function 'hints' on the screen while I type. I am that sort of person who remembers the 'ideas' better than the actual syntax and really benefits from knowing right then and there that the code I typed was correct, not having to find out a minute later at compile time. And then to have to try and figure out if I just misspelled something, or if I truly made a conceptual error due to a misunderstanding of proper use of the language. Code completion and hints have always saved time on this for me.
I know some people may look down on this and say the feature is unnecessary if you know what you're doing, but I never claimed to be better than anyone else.
I may have to just give up and try and get OS X running on my PC. Which doesnt bother me in the least, just the rebooting to go back and forth to windows. I've tried to run it virtualized under VMWare but XCode kept crashing :( That reminds me I am going to google 'leopard vmware' and see if any progress has been made in that area.
Another problem in designing a full code-completion system with objective C is that the syntax is a little more forgiving, I dont know the exact technical term (strongly typed?) it is much harder to say exactly what sort of object belongs in a certain parameter and ANY object can be sent ANY message whether it implements that function or not. So you can spell a function name wrong, but it doesnt necessarily mean you made a syntax error... maybe you mean to call a function of that OTHER name and you just want nothing to be done if the function is not implemented by your object.
That's what I would really like to see for Objective-C, is an IDE that once it notices you are sending a message to an object, it displays a list of methods and function definitions that the object is known to accept, and walks you through filling in the parameters.