I distinctly recall my txt experience on the 2007 iphone being one where I could just type and not need to worry about predictive text not doing it's job. At the time I was working for a company who's product was a federated text client. And the results were good. I could type sentences without looking and when I looked down, most of the time what I expected was there--no correction needed. Perhaps this is where predicting sentence structure or personal idioms does not work as well when applied to too wide a scale.