Having it sync without any effort with my laptop means that I have all my contacts, appointments, photos, music, movies, that I want with me.
I'm running mine unhacked so I'm looking forward to 3rd party stuff later this year after Apple releases the SDK this month to developers.
I'm using
webapps a lot more since Apple made it so that you can make them buttons on the homescreen. One web app that I've come to use a great deal is
tadalist. It's just an online todo list but by having the webpage open on the laptop that I keep in the kitchen and adding items to it I always have my grocery list with me. I've also got a bunch or other todo lists.
Haven't my email with me always is fantastic too.