I believe the main uses are:
1) Backup of i devices
2) Storage of music, video, TV etc. bought through iTunes or matched using the match feature
3) Storage of data in applications on i devices I.e. Pages and numbers
4) Sorage of pictures taken on iPhones (photo streaming) which are on the iphone camera roll. They automatically appear on iPads, PCs and Macs.
I do not believe that it is for storage of absolutely all data. I tend to use it mainly for keeping calendars in one place on my iPhone and iPad and for storing a copy of of other data on my iPad/iPhone.
We get occasional calls from people who have lost data stored in the icloud (I'm sure others have issues too, just not come across them yet) but it's usually limited to the odd picture or contact. That said, I'd always do a "local" backup every once in a while anyway if stuff is important.
I use iCloud to back up our phones and iPads. I also use the photostream option, but I do not save documents in it nor have I activated iTunes Match which I think keeps your music in the cloud too and makes it available to all your devices. I've had no problems with it. (Makes setting up new iPhones/iPads a doddle.)
And dont forget www.icloud.com for grabbing a contact or email etc while out and about with no idevice on you.
Also (one of the most useful aspects) find my iPhone/iPad/Mac, very useful :thumbs:
especially if you have lost your phone down the back of the sofa and can't find it - just set to to ring from a computer