I went 11 years being as an extremely loyal Verizon customer. I loved Verizon. Loved their coverage, loved their customer service, hated their phones. When the iPhone came out, I called them monthly begging them for a phone ANY phone that could compete with the iPhone. Nothing. I finally gave up. How much do I love my iPhone? I paid a $325 termination fee to get my entire family share plan canceled at Verizon and moved us all to AT&T.
Honestly, if Verizon wasn't so tight lipped about the android phones (I actually liked the form factor of the Eris more than the Droid), I probably would have stayed with them. The androids are great phones. But even still, I am sooooo glad I switched. I love the iPhone. There are so many things that are great about it. As for negatives, the AT&T coverage is simply not as good as Verizon. I have occaisionally dropped a call but it has been rare. The other negative of the iPhone is the lack of multitasking of 3rd party apps. I think that will change with the 4.0 release.
Which brings me to the major positives. The iPhone is always evolving and constantly getting software updates. Apps are also aways getting upgrades. Android and Windows are software platforms loaded on dozens of phones where the iPhone software is made specifically for one device. Those reasons alone make it head and shoulders above any other phone. The OS and third party apps can be customized for the iPhone. Then there are little things that make it great like just due to popularity, there are so many accessories to enhance your mobile phone experience, car adapters, iHomes made specifically for the iPhone (and iPods), cases. I wanted a Disney case and there were several to choose from. I love my classic Mickey case.