And it is easy to control! After a few seconds of severely oversteering (and severely approaching a big old mountain), I got the hang of gently tilting the iPhone to control my flight. A little icon on the left of the screen controls throttle -- oddly, requiring constant taps to increase speed, despite throttle not really working that way. Weapons are fired with icons on the right of the screen.
The demo level I played was a simple enough matter of keeping enemy fighters (who were very far away and therefore tiny on the screen) in target lock until the icon turned red, then tapping the "missile" icon and watching them explode. For a bite-sized Ace Combat experience, that seems like enough. The graphical presentation is somewhere in the early PS2 era, which is to say very impressive for an iPhone experience.
I went through about ten enemy planes before my brief three-minute demo elapsed. Ace Combat Xi should be in the App Store sometime this winter.