Always assume "No" unless you have a bunch of established crypto researchers who've looked at the system and who are prepared to use it themselves.

People might say that "there are no posted attacks for privnote", but that just means the bad guys haven't told you they can crack your encryption. Since bad guys will never tell you that they can crack your encryption you must not assume lack of evidence of cracks is the same as lack of cracks.

(http://pablohoffman.com/how-privnote-really-works)

There's some stuff that is vaguely worrying about privnote.

Really, learn GPG / PGP. Help other people learn it. Fix the UI. Fix the documentation. Sign everything. Encrypt everything.