if you would define what you mean by secure/safe to use. Just about anything can be hacked, including telco computers, government computers, servers, etc given enough resources and talent.
FWIW, I am fairly security conscious, though far from fanatic, use GV, love it, and don't see any dealbreakers for me personally. Google has something of a rep regarding possible problems with personal info, but I don't see how that would impact google voice, directly. Sign up using ficticious (sp)data (that would slow down google itself.... but keep in mind that you would also have to have all your PHONE numbers using fictitious data also. Even then, the phone company would have no trouble IDing you, nor would any of the federal 3 letter agencies.
If you REALLY need telco security, be prepared to pay anywhere from $600 to mulitiple thousands for voice encryption gear. I don't think anyone can stop to NSA if they are REALLY determined to hear you. But you can make it hard enough so that they would never ADMIT it.
If you just need security from your SO, use reasonable computer security, don't eliminate the password requirement on the home phones at least, etc. I say computer security because one of the features of GV is you can view your messages online. If your SO has access to your home computer, you probably should keep GV completely off of it.
Hope this helps
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FriscoJack