I use my iPhone to manage personal appointments much as one would use their corporate mail server to manage meetings.
My wife and I have Gmail accounts. Our phones are both set to use gmail as an Outlook Exchange server. What that extra distinction allows for is the ability to send and receive calendar entries and accept or deny them as you would within an outlook client on your computer.
When my wife has an appointment she invites me to it and it appears in my calendar after I accept it, for example.
The limitations are that 1) your mail server (e.g. Hotmail, Yahoo, etc) has to support Microsoft Exchange and 2) the scheduler has to know that ahead of time. It's works really well with my wife and I though.