I used one for a few weeks. I thought that the navigation for internet was good but the screen was too small, eyes hurting afte 15 minutes, and keypad use limited when it comes to email. I would gripe about the small power on button and the sharp feel to the case.
I think there must be better devices out there.... I saw a guy using some blackberry device at the airport recently and he was banging out email as if he was on a full size keyboard!! I am using a touch screen Samsung Omnia at the moment... jury still out. I think the transition to a PDA is not easy but like video, microwave, DVD it will come to us all at some stage. Good luck.