Ive tried to use it but i keep going back to dropbox. I tried to use it to sync a pdf that's about 200 mb in size but the fact that it keeps downloading the entire 200mb and not the part it changes is a deal breaker. (it doesnt do this all the time but it does this enough to waste bandwidth, etc)
Technically, icloud drive was "available" since at least mavericks (though one had to dig into the icloud folder in Library folder. Dropping a file in the app's folder (i.e. goodreader) would cause it to sync to the iOS app itself. Even in mavericks, it would sync via delta sometimes or download/upload the entire file. Figured it would be fixed once official support came out in 10.10. Same deal.
on windows, it's even worse and will download the entire file each time.