http://www.apple.com/dotmac/
Personally, I use syncing for my calendar, contacts, e-mail settings, dock items, dashboard items, Yojimbo items and bookmarks to keep things the same across my three Macs. I also use the iDisk for storing documents I frequently use on multiple machines. And of course I use the e-mail.
I've also on occasion used the web hosting space, but I'm hoping to utilize it more in the future, especially now that they give you so much more space. I'm planning on starting to use Back to My Mac shortly, I just need to upgrade my work computer to Leopard (which I'm hoping to do tomorrow) first.
For me, the syncing and e-mail is worth $100 a year. The rest is all bonus for me, though I'm expecting to get a lot out of Back to My Mac, but I've already had .Mac for 3 years and am happy for it.
EDIT: BTW, you can often get .Mac for ~$60-$70 instead of $100 if you order it on Amazon. They stupidly discount the old packaging versions even though the box is just a throwaway. All you need is the code. And there's also a discount available if you buy along with a new Mac from Apple.
EDIT: Oh, also, I use the .Mac Backup program, both to back up some data to my iDisk and to backup my whole computer to a network drive. I still use it in addition to Time Machine and SuperDuper!, but I'm kind of OCD like that.