personally i think games on the iphone are a waste of time. I played the rock band demo, i played the COD zombies one, and they all boil down to the same problem that a touchscreen doesn't give you the physical feedback that gamers are used to from traditional controllers.
the rock band demo in particular just *ignored* some of my button presses - I know I hit the note, I know my timing was there, but still I got the kerchunk sound.
while we're on the subject, the whole reason I stopped playing guitar hero (apart from the really really really awful song selection, which it looks like this game is falling foul of as well - how many people have you ever met that are into playing guitar *and* below-average girl pop?) was that the game continually screwed me - particularly on songs I knew well - well enough to know that the timing isn't always 100% spot on, no matter how much the programmers would like it to be. So you have the choice of playing the 'note' as it actually sounds on the song, or playing it as it appears on the grid.
[/rant]