As an owner of an iPhone, Blackberry and ordinary predictive mobile I have to question whether this isn't a simple matter of each to their own.
I find writing text the quickest in the order shown in the first paragraph. Admittedly I may key text incorrectly more often in the iPhone, but the software corrects my mistakes in real time almost without fail. I also find handy shortcuts like double clicking the space bar adding a full stop and then a space a handy time saver.
I also find it a lot easier to type numbers and punctuation on the iPhone as it has them all clearly displayed and you simply press what you see rather than searching for the symbol on the small Blackberry and then remembering to press the ALT key first etc.
That said, touch typing isn't really viable on the iPhone, so you need to trust that what you are pressing is actually being displayed. This could be a problem for some users.