The other thing that’s a killer app for me is sharing calendars.
I needed a shared calendar for me and my OH to keep track of stuff we’d booked / planned. So I have my personal calendar which is ostensibly my work diary. That’s synced between Outlook at work and Google (I use an app called gSyncit for this, which is ace). We have a separate shared calendar that’s on my account and my OH has full read / write access to.
Both calendars sync to my phone over the air, and in the calendar app I can toggle visibility of either calendar on or off. OH has an iPhone and that’s synced to the same shared calendar using some iTrickery I neither understand nor care about. There’s also a ‘birthdays’ calendar which auto-populates from my contacts where I’ve entered their birthdate (so never forget a mate’s birthday again), and you can link other data sources (eg, Facebook, Twitter) to further add missing contact data. Oh, and a UK Holidays calendar listing bank holidays and such. Again, you can hide these and just flip them on when you need to check them.
Oh the phonebook app, you can change the settings to control which sources display. So, for me I only have people displayed in my phone book who are actual contacts, ie people I might actually have cause to call rather than the 200 Friends acquaintances on FB, but the extra data is still there to bolster the contacts list with information they’re adding themselves.