I can't speak to the actual security, since that'd require a lot more knowledge that I possess (and an audit). Open source can mean more trustworthy, but doesn't per se mean it's more secure unless someone has actually gone through and looked at the code. Still, it's always good sign.
I found a Hacker News post talking about it, and the Tenta folks give a little more detail about some of their plans and what-not (although that thread is from awhile ago).
At the end of the day, it's a question as to whether you trust them enough to use their VPN and DNS servers.