You can use either, both and local.
There is no law that says that you have to use one or the other or that you cannot store you documents locally if you use the cloud. Personally the fact that there is no Internet in the hills of Peru does not reduce to the value of Google to me in a big city with Internet everywhere. I use Linux and I happily use Google for my documents and email and cloud storage. Do I care for an instant that for 3% of my year I am away from Internet access? I can use LibreOffice, store locally and upload when I am back on line. I would like to see LibreOffice and Google formats be absolutely compatible. For all my documents and spreadsheets (relatively simple) they already are. You can use Windows or Mac in the same way. One day you computer dies, or is stolen or run over by a tornado. You buy a new one, log in to you Google account and there is you email as you left it. There are your docs, even the one you were working on when the machine died. You can then download the docs back to you local hard drive at your leisure. No mucking about with disk images and csv, mab etc files - instant back on and going. The uses of shared docs are only limited by your imagination. I help with the running of a small club, membership lists and email can be shared by all the committee and it costs nothing. I use a NAS as well. Using one does not mean you can't use the other. I buy my music from three different suppliers. I just go from one to the other looking for the best price. Again, just because you use one, you are not excluded from the others.
I do not use ChromeOS, I prefer Chrome browser on top of a simple Debian install running Xfce. Most of the functions of ChromeOS with a comfortable and secure background system for manipulating files, printers, connections etc. Call it ChromeOS Plus. I have an Android phone. Enter a contact in Gmail and a few seconds later it magically appears in the phone. Buy a new phone, log in to my Google account and seconds later all my contacts are back. Easy.
Apples iCloud is just a start. Online backup is good. Syncing is good. Instant restore email and online docs are even better. Apple and Microsoft and Google will all match each others clouds in time. Competition is good for the end user.
