I have never owned an iOS device before, but i have to admit they made a better job with the UI than Android. The smoothness of it all just amazes me. But there are two things keeping me from getting an iPad:
The first is the horror of iTunes, which has to be the worst piece of software in history, at least on the PC, and the lack of a universal file system. I used to have an ipod, but i ultimately parted with it because it could only be synced using itunes. On android you just drag and drop, and open the file with any app that supports it.
The second reason is the lack of a "back" button. This is probably because i never used an ios devecie for a longer period, but every time i play with someone's iphone/ipad i get stuck somewhere in apps, not knowing what to do next. Every app has a different way, some have a back button on the screen, some are gesture controlled, and some are just plain stupid. I once asked the owner of an iphone how to get back to the previous screen. He went back to the home screen and restarted the app. It's probably just his stupidity, but talk about intuitiveness.
Our former editor-in-chief Adam Pash often complained about the inconsistency of the back button on Android, so clearly you can take the same issue and consider it a deal breaker on either OS.