I use mine a lot, but then I have several relatives with Macs, and we swap files fairly often. The .mac membership includes virus protection software as well, which is nice. Also, you can use Apple's free Backup software to automatically back up important files to your iDisk on a regular schedule.
A lot of people are annoyed with Apple, because this service used to be part of iTools, which was free. Now they're charging for it.
In my opinion yes, it's worth it for me. But I urge you to read about the features and decide for yourself.