Tell me if I'm odd one out, but I've never really taken to iDisk. I love MobileMe and appreciate the seamless syncing of calendars and contacts among all my Macs and my iPhone. And I love the way it keeps all my OmniFocus data in sync between Macs and iPhone. But I've never really warmed to iDisk. It mounts, it unmounts, it leaves a copy here there and everywhere. It's never quite ready when you want it. In short, I find it a nuisance.
Dropbox, a free alternative, is what iDisk should have been. It is a folder, linked to the Dropbox cloud, and can be placed on the desktop (which is my choice for easy access) or anywhere else in the home folder. All the contents of the Dropbox folder are synced immediately between any number of Macs or PCs and, newly, with the iPhone. It's really quick.
It's not just a matter of having your data backed up offline. I now use Dropbox for all my current data files, except the biggies like music and photos, so I always have the latest versions with me, whether at the desk or on the train with my MacBook Air. And for those apps that mainly use one data file (such as my Moneydance accounts), Dropbox offers an ideal solution. I can update my data on the desktop before leaving home and be sure than the file will be current when I open my accounts data on the Air later in the day.
Take a typical use for Dropbox: The keychain for 1Password can be stored on Dropbox and it means that all your Macs have the same information whenever you try to long on to a web site or want to check your bank account details. When you open 1Password on any of your Macs you are connecting to the same data file. (Note that this does not extend to the iPhone version of 1Password which must be synced via a common wifi network).
Most people these days have more than one Mac, probably at least one desktop and a laptop, and an application like Dropbox ensures that everything is up to date. There's never a suspicion that you've left that important file at home.
Security, too, is exemplary. Data is encrypted on upload and download, so the files on the cloud are not accessible to third parties. But since the data is held in local files you end up with effective backups of all your data on your other Macs and on the Dropbox cloud; even on your iPhone. If the worst happened and Dropbox lost all your data (unlikely, but look what happened this week to Microsoft and the T-Mobile Sidekick) you still have your stuff on all your devices.
Dropbox keeps copies of your deleted or amended files so you can go back and restore something you thought you'd lost. It's like a portable version of Time Machine, available wherever you are. This data retention is time limited in the free version but extended if you take a subscription.
You get 2GB of storage for free, and that's quite a lot when you think about it. I upgraded to 50GB for $9.99 a month but so far have not even exceeded the original 2GB. I paid mainly because I think so highly of this product that I felt I would like to contribute to the development: A rare sentiment these days.