Having gone the *other* way, I can say "Your Mileage May Vary" .
My iPhone is a great entertainment device. On a trip where you dont want to lug a laptop, it is quite good for a movie, and the iPod functions are obviously great.
What it didnt do for me, was make my business life any easier. I use tons of minutes, e-mail and text during my work day. Given AT&T's motto of "we're not happy until you're not happy", the assorted 3G issues even in Southern California, and the $150+ bill added up to trying something else.
For me, Sprint Everything and a 8330 works better, and I can type when I am wearing gloves. And add memory capacity without out having to buy a new phone, cut and paste, MMS, etc...
In the end, it doesn't matter much what anyone says, until you give it a try you won't know for sure.