Specifically I like the email that can be accessed using a real client *or* a web interface (very handy when you're on the road)
I like the ability to use isync to backup my personal information (keychains, address book, preferences, etc) to a nice safe network available volume.
I like the idisk/webpage/photo sharing stuff.
If you have a properly managed machine on a static ip address that gets properly backed up then most of what .mac does you don't need. But if you don't, its really quite handy.