While the worth of .mac may have been dubious when it was first introduced (I still think $100 was worth it when it was introduced, but that's my opinion), .mac has steadily become more and more useful.
First off, iSync. OMG this is *SO* useful. I have used this so often through .mac that it isn't even funny. While iSync is marginally useful on one computer, it's really nice with .mac. That's because it syncs your contacts in Address Book, events in iCal, and bookmarks from Safari across multiple computers. I sync regularly so that I can have the latest information available everywhere. If I add a bookmark here at work (where I'm typing on an old B&W G3 tower), I simply iSync at the end of the day, and I get the bookmark back at home when I iSync there. If it's the school year, and I come home and need to use a comp, I can grab my mom's iBook or my dad's PowerBook, fire up iSync, click the Sync button, and in a few minutes all my calendars, contacts, and bookmarks will be there. I never need to worry about if my contact has the wrong info, or if I'm missing a bookmark. I can't stress enough how SO USEFUL this is. If there's one thing to sell you on .mac, it's this.
Second, the iDisk is indispensable. Being able to access my files from any Mac in one keystroke (Command-Shift-I in the Finder), and even through Windows machines is excellent. I have used it many times to shuttle things to different Macs, where it would've been annoying and/or wasteful to burn a CD (I don't have a floppy drive) or e-mail it.
Third, mac.com e-mail. Yes, you can probably get free e-mail elsewhere, but I signed up with iTools when Apple first introduced it, and didn't want to have to deal with changing e-mail addresses. This point is moot if you're just getting .mac, but I for one find mac.com e-mail very reliable. Contrary to all the user reports on MacNN that I've seen in the past year or two, I can only remember one brief outage with my mac.com e-mail in the history of my (what is it, now?) 3 years of use of mac.com/iTools e-mail. .mac's web e-mail is also very polished, and it's the one and only one web-based e-mail that's actually NOT annoying to use.
Fourth, .mac Bookmarks. This is the least useful to me at this point because everywhere I go I have access to a Mac. But I might not in the future. Originally when I started here at work this summer, they didn't have a Mac for me to use, and .mac Bookmarks weren't available either. I was never able to transfer over my bookmarks from my Mac to the Windows machine, despite my own program that converts Safari bookmarks to other formats. .mac Bookmarks would have been very useful in this regard.
The other features of .mac are marginally useful. Backup seems cool, but I've never actually used it that much... it just wasted my iDisk space, and I didn't feel like wasting a few CDs. Free anti-virus software is always nice, but I also get free anti-virus software through Stanford, and I'm actually careful enough that I would be comfortable not installing any anti-virus software at all; I haven't gotten a virus on my personal machine since 1986 when I started using a Mac. All the free games and special offers are nice, but I rarely use them that often. I download them just so I can play them if I ever really wanted to, but I usually never do. iCards are very cool for the very few times I do use them; it's a shame that they're limited to .mac users now.
Despite the multitude of almost useless features to me, the first four reasons that I've detailed to get .mac practically sell it. I would buy it if that's all they offered, since those features are that good. I'm sure some people will appreciate the features that are useless to me, but I just wanted to give you my experience.
I think you should really consider getting .mac; it's been very useful for me. Note that if you're thinking about getting a new comp, I think you can get your first year of .mac for $69 instead of the usual $99, provided you buy it with a new Mac.