Perhaps I'm biased.  Unfortunately, people who purchase an iPhone may also be biased by post-purchase rationalization.  I actually have used touch-screen keyboards, especially the iPhone's, many times.  I'm not saying it's the worst experience ever.  It's far better than typing on a number pad with T9 or another predictive algorithm.  With time it would certainly be doable, maybe even unnoticeable to the user.  Still, it is far worse than even the worst tactile qwerty phone keyboard.