This is where AppCode comes in, it is developed by the same people as IntelliJ: JetBrains. With AppCode they basically built an IDE very similar to IntelliJ, but instead of Java it supports Objective C. Currently it is still in some kind of pre-beta stage they call EAP (Early Access Program), but already it looks very very promising.
At first I was very sceptical about AppCode, because I didn’t believe in an IDE not part of the standard iOS SDK package. Because the power of how Apple distributes new releases of the SDK is, that the tools are also updated with every release. So you will never run into compatibility issues. A third party IDE would probably have a hard time catching up to the latest iOS version every release. It will quickly fall behind.
I suspect the guys from JetBrains have also foreseen this problem and came up with a very clever solution. They basically built the entire IDE “on top” of XCode. This means:
AppCode doesn’t have its own project file format, but instead it opens existing XCode projects
It behind the scenes uses the XCode compiler to build your app
It leaves editing NIB files (Interface Builder) entirely up to XCode
This is very cool because this way there is no risk or effort involved using AppCode instead of XCode. Just install AppCode, open your existing XCode project and you can start using it! If you find something that really only XCode can do (for example distribute to the App Store) you can still switch back to XCode to do that task.