I second schweb's suggestion, but as someone who has been with MM in its various incarnations since day one, here's a short summary:
The biggest deal with MM is *convenience.* I can put up a photo gallery, host a video, make a website or publish a podcast without even thinking about it, and I never run into "bandwidth" gotchas or hidden charges. Publishing to MM is integrated into most Mac tools and very easy.
I like the ad-free, tracking-free email. Clearly it's not an issue with most people, but it is for me. I also get an easy-to-remember email address that hasn't changed in over a decade, works on any internet connected computer and uses IMAP which I require for my travel. GMail is a strong contender if someone is looking for email alone (and doesn't mind the ads et al).
The iDisk may not be the fastest way to upload and share material, but it's the easiest. Dropbox is a nice alternative if that's all your looking for, but it's very limited (2GB) without paying. I get 20GB as part of my membership.
I could go on, but the bottom line is that MM is a good value for money *if* you are going to use all or most of its features. Most of it can be recreated on some level by a half-dozen or so free services, but that's still something of a hassle by comparison with the easy-to-use, integrated approach. And I say this as someone who regularly uses GMail, Dropbox, Flickr and YouTube as well.
Finally, word is that MM is about to get a big overhaul soon, including (so they say) automatic cloud backup of music and app purchases (maybe photos as well). Could be interesting, but I already feel I'm getting my money's worth (I do often renew my membership through Amazon rather than directly through Apple, which often saves quite a bit of money on the annual fee).