I have used Unity and worked with some of their people, here is what I can offer:
Unity is a visual editor and as mentioned by ManiacDev you can use C#, Javascript, and Boo.
You can do a lot without coding a line and it outputs a file for you to compile in xcode. That's pretty sweet. It is very much WYSIWYG - but for some that leaves a lot to be desired.
Examining this further:
If you did want to use your own Objective C classes and code and bring them into Unity this would require purchasing their Pro version. Personally - I wouldn't go the basic license for this one reason alone. If I want to use my own server or anything else I have already coded in other projects of course I would want to take advantage of the work I have already created and not rewrite more code in C# - for the iPhone this makes no sense at all considering the uniqueness of the device and the other possibilities that could be imagined when having full access to the SDK.
The other thing you might want to think about is that their logo will appear with the basic version - if you don't care and you want to use their tools - the basic version is a great entry point. I personally do not mind their logo at all but again not having the full SDK at my disposal is not a place I would like to be locked in when you want to see your full creative potential.
Hope this helps,
Matthew