I can't comment on Cocoa. But if you know C++ Qt is a dream. They have amazing libraries that make up for the shortcomings of C++. It also makes native look and feel cross-platform development a snap. Google Earth is built in Qt. It has a dual license, documentation + community are great. Can't say enough good things about it.
If you want to create Mac OS X apps, forget about multi platform stuff - your customers do not care if your app runs on other platforms, and they will ignore your app if it does look and behave like a great Mac app.
It is a universal Gui API. So on OS X it api's to carbon (soon to be cocoa.) On windows it api's to mfc and on linux to xwin. So yes, it has exactly the same look and feel as a mac app or whatever platform you choose build on. Once it's compiled it's just a bunch of native system gui calls so there is no way to tell the difference. You've probably used Qt a million times and not known it. Opera and Google Earth are both Qt apps.
Even if I was only releasing on one platform I would still use it b/c it is so simple and powerful. And what's the harm in having your app available to the other 90% of computer users?
Opera and Google Earth are good examples of such apps - they suck. Both have non native and ugly user interface. So if you your app does more or less what other apps do, but looks like this, people will prefer the native cocoa apps.
Agreed, they have ugly Ui's but that's not Qt's fault. When you want an "Ok" button, a dialog or a check box in Qt, it's calling the EXACT SAME function that get's called when you do it without Qt. So by definition it is native. You should check it out. You might change your mind.
I can't comment on Cocoa. But if you know C++ Qt is a dream. They have amazing libraries that make up for the shortcomings of C++. It also makes native look and feel cross-platform development a snap. Google Earth is built in Qt. It has a dual license, documentation + community are great. Can't say enough good things about it.