iCloud looks great on paper, but the effects of choices you must make to enable it are not obvious, so, as a busy person with more than enough troubleshooting to do, I am shutting it off for now.
No replacement for iDisk yet.
I'm probably one of the few people that use iDisk, but I do use it. I store my Papers library on it, and a variety of assets I use at work and at home. It was a great little server, without any hassle. According to this technote, I have to save my files and get ready for it to disappear.
I never used the iWeb or MobileMe Gallery features, but it looks like all of that is disappearing also. iCloud is neither a hosting service, nor an unstructured file-server. Documents can be stored by apps, but probably not dragged there by you, as far as I can tell.
With the advent of iCloud, your AppleID takes on new importance. iCloud's functions include backup of your purchased music and apps, backup and syncing for email, photos, calendars, and contacts. Each device can utilize some or all of the available functions.
The problem is, many of us have accidentally acquired more than one AppleID.
If you purchased your Mac or device online, it is likely that you used one AppleID to purchase it, then created a MobileMe account once it was up and running. That MobileMe account is likely what you use to sync data across devices - at least until now.
If you have been using MobileMe to sync data across devices, and another AppleID to purchase apps and music, you have a problem: the two accounts cannot be merged into one iCloud account. In several cases, even though I have a me.com address registered on one or the other device, it wants me to make another one! I am not even certain what result the choice of one or the other AppleID will have, but experimenting with it is likely to result in data loss.
Time to upgrade to Lion? I think not...
Where MobileMe worked across all devices, iCloud will require OS X 7+ on desktop Macs. I tested Lion pretty thoroughly a couple of months ago, and decided not to use it for reasons ranging from software incompatibility to apparent bugginess and network problems. Perhaps by the time MobileMe goes away I'll be able to switch, or perhaps I'll simply have to live without iCloud on the desktop.