A lot of my Hong Kong friends text using Roman letters, because it is faster. There's a popular hybrid SMS language that uses a combine of Chinese characters and the Roman alphabet to express things in phonetic shorthand.
For instance, the phrase "a little bit" might be expressed using the Chinese character for "one" (a single horizontal slash), followed by the letter "D", which sounds like the spoken words "Yat di".
This SMS shorthand also seems to be prevalent on Chinese language blogs.
Anyway, Rory is right--even though my Chinese writing skills are bad, I have no problem texting using the five-key stroke system. Whoever invented that predictive text program is a genius.