I find it quite useful and think probably my most used feature is the e-mail, even though I could just use Gmail I suppose. I've been using iWeb for a little bit now and with the new domain forwarding thing, I'm really happy with how my website looks these days (in my signature).
When .Mac was first around they had an offer on a blogging tool called iBlog. I still use that as oppose to iWeb or anything else and even though it has it's problems, it fits in so well, so maybe the reason I couldn't give up .Mac now is because I'd lose my blog.
For a couple years I had only one mac, but still used a number of the features, having two macs has just meant they both have the same info on them, which is so much better.
Backup does a weekly backup of my personal data such as keychains and preferences to my iDisk, so I never have to worry about losing any of that.
My gallery is also quite nice for keeping photos and movies, it has worked very well updating daily from different foreign countries with new photos, so that's a plus.
You can use iChat with an AIM account or free .Mac account, but encryption between other .Mac users is nice and probably satisfies a few security buffs.
Between my website, blog and gallery I don't think I'd ever use the bandwidth they allocate to me every month, the best I've ever done was in September when I hit 500MB for the whole month, out of a total 100GB mind you though
While many of the features you could piece together in stand alone products for less, its the fact that it's all just the one product that's useful, and it's all the things in the background that I never think about, but they just work, luckily.
