I upgraded to iPhone 5 from the iPhone 4, and I'd say the biggest improvements to my experience have been
LTE
Siri (really, improved Voice Control)
Turn-by-Turn directions
(in that order)
-------
1. I use Siri daily when I'm NOT in the car:
-To call people: Holding the button and saying "Call my wife" is faster than unlocking, tapping phone, tapping favorites, then tapping my wife's number.
-To use Find my Friends to see when my wife gets home / leaves work, etc.
"Where's my wife?" and a map pops up showing her location. Wonderful.
-------
2. I use Siri daily when I AM n the car:
-Text or iMessaging:
"Tell my wife I'm ten minutes late."
-Send quick reply e-mails.
"E-mail person x: Sounds good."
-Launching and using Apps.
"Launch Audiobooks" starts the app and automatically begins playing my audiobook where I left off. No touching required.
-To make notes to myself: I'll touch to pause my audiobook, then say "Siri make a note:" and then dictate a quote from the book I'm listening to. (This sometimes works well, sometimes not. But it is handy to have the ability to take notes on an audiobook, hands free.)
-For directions: "how do I get to xyz?" (this auto-starts the turn-by-turn directions. Very handy.)
-------
My critiques/complaints:
For the way I use it, I would have been happy if Apple had simply expanded the list of Voice Control commands to include the ability to launch apps, get directions, and play music. The context flexibility of Siri is nice, I guess, but once I figure out what phrasing gets me the result I want, I don't change how I say it anyway. I could have easily learned whatever syntax Apple wanted to use for Voice Control. And it's a pain that Siri doesn't work when not connected to the internet.
I used voice control alot on the iPhone4 to listen to music in the car. My annoyance then was that you could tell it to play songs by a certain artist, but you couldn't tell it to play a song by title. Still don't understand why, since for both the artist and the song title it has to interpret my voice and compare it to one of the song's text fields. Siri allows me many more options, but I wish it didn't require the internet.