For my in-game editor I wanted to create buttons and some textfields, however I did not want to re-invent the wheel and also did not want to complicate switching between views since the UI for the editor will not be complicated at all.
I know that UIKit elements should not be overlayed on opengl because of a performance hit, but it doesn't seem to effect it that much so far. It's pretty simple to create a UIButton and then just add it to the window: [window addSubview:b];
I created a helper method to quickly create consistent looking buttons and I'm using some iPhone images I extracted from the sdk. I'm quite happy with the cleanliness of UIKit, I must say.