A while back I told the JetBrains guys that I'd do an overview of the AppCode UI, highlighting the areas where it could be improved. It's taken a while as I've been busy, but I've finally written it up. It is by no means comprehensive, and some of it is somewhat subjective. Hopefully people find it interesting and useful and it helps start some discussion about the UI.
At the moment I don't see AppCode and Xcode been too far apart functionality wise. Xcode is better in some places (eg code completion) and AppCode is better in others (eg refactoring). The one area that Xcode beats AppCode hands down though is the UI. Prior to Xcode 4 I always said that "Xcode had the nicest UI of any IDE, but that that was like saying it was the nicest smelling dog turd". Xcode 4 actually made a huge leap forward and is not only really nice from a usability point of view, but it also just nice aesthetically as well. Hopefully AppCode can eventually compete with Xcode on UI as well :)