Just upgraded to iOS 6 on my iPad, and as far as I can tell all of my apps are functioning well. The only MAJOR drawback I can see is Apple’s new maps app (and of course the map data as used in Hill Lists).
They have replaced the fantastic maps and satellite images from Google with their own, and I can honestly say they are nowhere near as good. They might work for a sat nav app like TomTom, but for Hill Lists … not happy.
You have been warned!
UPDATE 1: there is a minor problem with Hill Lists where the maps do not show red pins, at least initially. This is a race condition in my code and the fix will be in my next update (already submitted).
UPDATE 2: all my apps are working well on the iPhone 5 in letterbox mode. The new screen resolution will be fully supported in my next update(s).