I switched from the Treo 680 to the iPhone 2G, now 3G. I'm also a Mac/MobileMe user, and I love the "it just works" aspect.
Some people have time, inclination, aptitude, and even strong desire to fiddle and tweak and really hone every aspect of the device to their exact needs and liking (hi Boss!). Not me.
iPhone does 90% of what I need right out of the box, and I don't need the other 10% badly enough that I'd spend hours getting it (again and again and again).
(Sorry, organizing some thoughts with this thread, nothing to do with my pending Treo Pro review -- honest!)