The only "cloud" service I guess I really use is Netflix's Watch Instantly thing, and that is only because it came free with my Netflix account
And, to tell the truth, I'm never sure if the movie I put on my queue today will still be there tomorrow to stream, given the weird rights/expiring situation that occurs
from time to time.
If books were like that, it would drive me batty, because I re-read many of them on a consistent basis.