Well, have had my iPhone 3G since it's release now. This is the first iPhone I've had, I didn't have the first one, which looking back now, not sure why I didn't really. I was more into Windows Mobile devices at the time, so never really gave it a thought.
My first impressions; so easy to use, the interface is very consumer driven, unlike Windows Mobile devices, which try to replicate the Windows desktop interface which makes them a bit more complicated to use. Having used a TyTn II for a while now, it's no loss to have not quite so much configurablity available to me, it seems the more I changed settings on the TyTn, the less stable it becomes.
Key Features: Wireless is very seamless, nice how it connects without any intervention to previously used networks, email setup also very easy to do and works extremely well.
Missing Features: well, MMS would have to be one. Also, ability to use songs on the device for alarms, etc would have been nice. Battery life not as good as I believed it would be, I'm probably getting about 2 days if I don't use it too much... Finally, I do miss TomTom, must look to see what's available on the iPhone, it probably was my main application on the TyTn and got the most use (second only to email!)