for 99$ you can buy a huge 2-4 gig usb drive and back up to that. .mac is seriously overpriced for what you get.
You can use it for email too, but its way to expensive.
i think the website for .mac is
Apple .Mac
the idisk can be opened from a finder window. double-click on the drive on your desktop and in the window that opens in the top right side should be something "Idisk"
if its not there, then hit command-, and go to the sidebar prefs, click on the idisk thing.