I think it's worth it. I work on a PC during the day, and having access to my Address Book, which is tightly matched to my .Mac email address, well... I really like that. And the BackUp feature is nice, too. Since getting a new iBook I've been able to restore from my .Mac account and bring my Safari and Address Book up to speed real quick.
So if you need access to email and your address book while on the road and such, I recommend it. The Backup thing is kinda worth it, too, though more tech savvy users would just use server space somewhere else.