I've never used it before. Is it better than Dropbox? What do you use it for? Can you select what goes to iCloud if space is limited on some of your devices?
iCloud is great for syncing contacts and calendars, but IMO pretty limiting for syncing documents and not nearly as good as Dropbox for this purpose.
I use iCloud for contact and calendar sync as well as email, then Dropbox for document sync.
If you exclusively use Apple apps like Pages and Numbers on bot OS X and iOS for documents, then iCloud is okay.
Cool. What about photos?