Having used a Palm phone for the past two years, I can tell you that the integration is great but the size of the phone is a problem. A functional PDA needs a 3-4 inch screen. I was seriously looking at the Treo 600 earlier this year, but the screen was just too small for me to be able to effectively use it as a PDA. Hell, the buttons were too small for me to even dial a phone number.
Anyway, I'm moving back to a seperate PDA and phone. Bluetooth makes the integration a non-issue in my mind.