InfoWeek reports on a User Centric survey of iPhone users on the device usability
The learning curve was reduced on the iPhone but Google Maps was difficult to use according to some survey respondents
Safari Web browser was slow and they were frustrated by the browser
User Centric found using the outgoing and incoming call feature, visual voicemail, contact management, and taking and e-mailing digital pictures were all deemed successful by the iPhone users.
Participants found that text entry was much easier on the touch screen soft-keyboard of the iPhone compared to standard multi-tap text entry
users preferred the horizontal keyboard to the vertical keyboard
liking for the iPhone's horizontal view
As expected, users of the iPhone, which also has iPod capability, found selecting and playing music to be easy.
"The percentage of users actually utilizing all of the features that a smart phone offers was higher than other phones we've tested. The iPhone was more intuitive than other devices."
See here for details of the study