It depends. If you have a healthy social life, healthy. Just keeps you going on track, sometimes hearing things aloud make everyday actions easier. If you don't have much of a social life, unhealthy because you are talking to yourself and don't feel the need to talk to anyone else.
It depends on why the person is doing it. If the person thinks, they're talking to imaginary friends or people who aren't there. It's probably not a healthy thing. If a person is talking to sort out a complex problem or to help them focus, I don't think it's health or unhealthy. It's all about what is effective. It's probably not good during a test since it might be considered cheating if the person is giving away the answer. That could also be a symptom of a problem if the person can't control it.
Depends on whether you like yourself or not ! If you are your own best friend and can talk to yourself like a "Dutch uncle" (look it up!) then go for it! But you just "shoot yourself in the foot" if you're your own worst enemy!