I have tried many touchscreen devices and always go back to numerical keys. Here’s why.
I can touch type on a keyed device. A touch screen has no tactile feedback making it useless for non-sighted use.
With a keyed device, the numeric keypad is by far the most gracious design. It constrains the input area sufficiently that I can operate the device using one hand and I need very limited movement in my thumb to operate it. Held in either hand, the side-keys on my Nokia E51 are in easy reach of my fingers and the main keypad sits nicely under my thumb. With a touchscreen, you need two hands if you are to avoid very awkward stretching and balancing.
After a little time, T9 input is faster than QWERTY. Because I only have to move among 12 keys, I don’t have to look and optically digest an entire keyboard. Tengo provide a QWERTY version of T9 for Windows Mobile devices (I think they may have gone under).
Constraining input to fewer keys removes the awkwardness of a multi-faceted input UI. There is no doubt that the back key on my device will always perform back functions. Even on the iPhone, this is performed by an on-screen button which isn’t always in exactly the same place. This is more true of the ’send’ or ‘green’ button. For touchscreen devices this is often anywhere on the screen which means you have to hunt for it.
Keys offer me precise control. I know when I have pressed the key and I get immediate feedback. For a touchscreen I have often experienced a ‘dead touch’ where the screen hasn’t registered my finger.