In order to start using it, you need to be online, but once you've successfully logged in, you can use it without connection for several days.
It won't save (back up) during that time, so your projects are stored locally only.
Without connection, you won't be able to send your files to other CC apps.
Short answer is yes, but with exceptions.
ProTip: Getting away from internet connection for a while? Log in to all of your CC mobile apps separately before you leave WiFi access.