Are you talking about more subjectively usable (maybe you're some kind of picky cloud storage critic, and will be the Judge of That), or objectively usable (as in, you can store things and retrieve them)?

If the latter I have seen people saying you can do the basics well enough with pCloud.

Define “useable”, there are plenty of options but no clue what “useable” means to you. Does that mean specific platforms (iOS, Android, etc.), does that mean automatic sync?

Personally I like using my own cloud compute (Digital Ocean Droplet, AWS EC2, etc.) and then setup syncthing.