I use Dropbox, and I have all of my files synced on not just my Apple products, but also on my Windows boxes as well.
Note that iCloud is supposed to also work on Windows Vista and Windows 7.
That said, I'm going to keep some files in Dropbox even after moving most to iCloud, because I want to access those specific documents (mostly OpenOffice stuff) from a Linux (Fedora Core 14) box as well.
Dropbox is very nice for what it does. But it's still a folder in the filesystem. But with iCloud Apple has gone far beyond Dropbox. It syncs music, apps, iBooks, and files. And iCloud does this in a way that makes the filesystem disappear. And that is one of the primary ideas--to make the filesystem disappear.
By the way, iCloud will sync music between Macs and Windows boxes. But since I got rid of all my Windows boxes, I really don't need that. And once iCloud goes live, I won't have much need for Dropbox anymore since I have already made the transition to an all Mac shop.