I can live without it, as a driving aid it's hard to beat, that said I have a Bluetooth interface on the car audio.
So I get GPS instructions through the car speakers from either Google Maps or Fruit Inc Maps - no £130 SD card to buy annually. If there is an incident, I have in the smartphone, a camera with flash to document damage, other people, licences. Were a call to road services needed, I can tell them the exact coordinates of where I am.
For swatting mosquitos in the dead of night, it (the screen) illuminates the blighters without spooking them like a regular torch... Whack!
Working mobile, I can ask the office "Shall I replace this lock (pictured) as well?". In my unscientific and amateurish opinion, the holiday snaps taken by a modern smartphone are more than good enough for most people and the addition of another device (the digital camera) is just gadget addiction.
I like the option of contactless payments using a smartphone, not as the only option, but a potentially handy one. If I go into the hardware, I can with the phone, show the staff an item listed on their website and say "Where will I find this?". The news, videoconferencing, movement tracking is just frippery to me. And I love the fact that if I open up Fruit Inc's "Find Friends", it says to me "No Suggestions".
The calendar with alerts and ability to set recurring reminders is useful, sharing events can be handy although this is often restricted to like forms of operating system. You can also overlay other calendars, this can save a faux pas based on cultural infamiliarity. Have to say also, the Fruit's inbuilt spirit level is scarily accurate, I have screenshot inadequate fall on a roof and sent the data straight to the office - things like this help to shorten the time between identifying problems (or their root causes) and gaining assent to remediate. Happier people, faster, and invoices turned around faster.
A final note from myself on this, the iPhail 4 does most of my listed activities!