I use it. The integration with the OS is what keeps me too it than anything else (besides the fact that is has been my email address for a long time). .Mac is integrated with finder, mail, ichat, ical, iweb, system preferences, and more. ANd what you really have to think about is that the price is only 8.25 a month!
Finder can store an offline copy of your iDisk for when you don't have internet access, Mail makes it easy for .Mac users to get email (only have to type in username and pass, it knows everything else), iChat works seamlessly with .Mac, with the added benefit of encrypted chatting, iCal lets you publish calendars to .Mac, iWeb gives you one click publishing to .Mac, system preferences makes it easy to transfer and sync data between two (or more) of your computers.