When I was "upgraded" by work from "flip" to "i" about six years ago, I dreaded the change. I thought the iphone would be fun for a week, then go by the wayside like most gadgets.
Boy was I wrong (speaking only from my personal experience, your mileage may vary)! I loved the darned thing, and would never go back. When I work on my TR I can look up something without getting up, I can snap a quick high quality photo for reference. And I can stick the phone in a hole and take a photo of something I can't see. Grease cleans off with a simple alcohol or windex wipe. When travelling, I have a GPS and travel agent in my pocket.
However, I hate the telephone feature! My fingers don't register on the sliding screen half the time, so I miss a few calls only because I can't answer them fast enough. Worse if I'm wearing nitrile gloves while wrenching. Give me a flip phone for calls, and an iphone for all the other cool stuff it does.
(Side note, given the photography folks on the site, I realize "high quality iphone photo" is relative... it cannot take cool helicopter shots, for example!)