A usability group called User Centric has taken a look at the iPhone's on-screen keyboard to see how it compares against phones with regular keypads and QWERTY keyboards.
For this test they used 20 participants which were giving the task to send 12 standard messages with the phones. The researchers found that entering text messages with the iPhone takes about as long as with a normal multitap phone but that it's about two times slower as on a phone with a QWERTY keyboard.
“For QWERTY users, texting was fast and accurate. But when they switched to the iPhone, they were frustrated with the touch sensitive keyboard," said Jen Allen, Usability Specialist, User Centric.
Also noted in the study is the fact that many participants hit the wrong keys on the iPhone’s onscreen keyboard and the errors were typically corrected by using the backspace key to delete characters one at a time because of difficulty getting the cursor inserted correctly in the middle of text. Only seven study participants figured out how to use the iPhone’s corrective text feature on their own. Improvement with accuracy while using the iPhone after 30 minutes was noted, but the difference in speed between QWERTY phones and the iPhone persisted.