Am figuring out Sierra on a "new" (2015 MacBookPro refurb) computer. I DID NOT sign up for iCloud Drive during setup, but apparently iCloud (?basic version) is activated anyway. I don't have a tablet or iPhone. Have been using Dropbox, which keeps copies of files on BOTH the cloud AND the local computer. (Read instructions on iCloud that stated it removes docs from the computer without maintaining copies locally. Bad idea.)
MY QUESTION: Have read method for disabling iCloud, but interested in opinions.

iCloud *can* move items off your system, but it doesn't have to. You just make sure the settings don't let it. Then it works like Dropbox.

That said, you can also not use it at all in two ways. One, never put anything in the directory. Two, uncheck iCloud Drive in the iCloud pane of System Settings.

If you want to use iCloud but not have it move anything, leave it active in the iCloud pane and make sure you uncheck "Desktop and Documents folders" in the options and leave "Optimize Mac storage" unchecked as well.

Of course, with a lousy 5 GB for free, unless you are willing to pay for more it's probably best to just turn it off.

Thank you. Sorry to say I don't trust Apple for this kind of thing. In the past, as I recall, they initiated special services, then eventually took them down and started something else & left some people without email or other function.
iCloud sounds useful for folks that need to coordinate tablets, iPhones, but am personally trying my best to keep tech life productive but simplified (if that's possible these days!) - as I'm the only "tech" at our house.