.mac in its current incarnation is really meant for a person with several computers who needs to share frequently updated information with a wide assortment of people.
I move between at least 5 macs several times a week—between time in a classroom, apartment, work, parents house, etc... and iDisk is ideal for taking notes in class on one computer and then accessing them for a homework assignment on another computer. iSync is great for scheduling an appointment on my work computer and then syncing it to my PDA on my home computer.
If I need to share a file with a mac user, it would go in my iDisk/Public folder. If I need to share a file with a windows user, it would go in my iDisk/Sites folder (to share over http). I do both of these constantly... and in a collaborative work environment, iDisk is priceless, and alot cheaper to purchase, set up, and maintain than any external fileserver. Plus: .mac doesn't have any bandwidth limitations like you would find on an intranet or if you were serving the files on some ISPs or most hosting companies.
I use the bookmark sync feature all the time... I can be researching something on the internet at work, and resume my research at home easily.
The web interface to the calendar, address book, and bookmarks has been helpful too. I frequently come in contact with computers running OS 9 or windows.
What's Wrong With .Mac and How to Fix It