I've subscribed to the Mantano Cloud service, and while it works as described, I've found Moon+ to be much easier to use. Moon "just does" synchronization, and I don't have to remember to select specific books for synch. It's also free to use with an existing Dropbox or GDrive account, which helps. I'm going to drop my Mantano Cloud subscription when it expires next year.
It's a shame Mantano doesn't adopt Moon+'s approach to sync, or Moon+ improve their rendering engine to be on par with Mantano or Google.