In any case, we've moved several projects to BTSync recently from Dropbox (for no other reason than to free up space on Dropbox for our personal files) and have been enjoying the service.
As a p2p encrypted protocol, I imagine it's much more difficult to eavesdrop on your files and would actually require a warrant to obtain.
I presume that's true for AeroFS as well.
And yes, they can give your data to the government. Communications to/from Dropbox are encrypted. But it is unencrypted on the back end. See http://paranoia.dubfire.net/2011/04/how-dropbox-sacrifices-u... for how we can know that.
The ability to detect duplication in no way proves the files are unencrypted (indeed this should be obvious from the fact that there is only negligible network traffic to confirm a duplicate! The bits can't be compared if they're not transmitted.)
It's the ability to serve deduplicated files that brings the service into question. Yet I wouldn't be surprised if there exists an asymmetric encryption method which permits decryption with one of several private keys – if so, secure deduplication is trivial: confirm the duplicate using a hash or comparing public-key encrypted versions; re-encrypt using both original and duplicate keys.
(And let's not even forget the ability to reset a forgotten password…)
And, if you're concerned about spying - well, it is closed source.
http://www.slashgeek.net/2013/05/16/host-your-own-dropbox-li...