Assume "the cloud" is always untrustworthy, and act accordingly.
Sync encrypted filesystems to the cloud. EncFS has some known issues, but for a lot of my stuff, where it's "write once read many times" it still seems to be OK from an encryption point of view - though it inevitably leaks file size information. On the other hand, it does play very nicely with Dropbox/GoogleDrive/BTSync - although for the sufficiently paranoid you need to consider the risk of the Dropbox/GoogleDive/BTSync binary running on your device having access to the unencrypted local partition... Using rsync or some open source sync tool might mitigate that somewhat (but I suspect if you've got data which you suspect an attacker might subvert Google or Dropbox to get access to, you _really_ shouldn't be trying to work out how to protect that data on someone else's storage - buy your own hard drives and deal with multiple copies and geographically separate duplicates yourself)
(because remember, there is no "cloud" there is only "other people's computers")
I attempted to use them for a while but found the upload speeds of their (proprietary and closed) clients to be utterly useless for anything more than a trivial amount of data.
Everything I found pointed to it being a client issue (the client was simply not uploading 90% of the time and my PC was more than capable of performing the required encryption and hashing operations far faster than my network connection could accept the bits) so I opened a ticket and talked to their support for over 6 months (!!!) and in the end they decided to just refund my money.
If you have a trivial amount of data, go ahead and use the $7, 30GB plan but don't fall for their 1TB plan, there's no way you can get close to using it.
Sync encrypted filesystems to the cloud. EncFS has some known issues, but for a lot of my stuff, where it's "write once read many times" it still seems to be OK from an encryption point of view - though it inevitably leaks file size information. On the other hand, it does play very nicely with Dropbox/GoogleDrive/BTSync - although for the sufficiently paranoid you need to consider the risk of the Dropbox/GoogleDive/BTSync binary running on your device having access to the unencrypted local partition... Using rsync or some open source sync tool might mitigate that somewhat (but I suspect if you've got data which you suspect an attacker might subvert Google or Dropbox to get access to, you _really_ shouldn't be trying to work out how to protect that data on someone else's storage - buy your own hard drives and deal with multiple copies and geographically separate duplicates yourself)
(because remember, there is no "cloud" there is only "other people's computers")