Zack barks a couple of times when someone comes to the door, but at the same time his tail is wagging, he's doing the kidney bean dance and licking the front door. So he lets us know someone is here but I wouldn't say he's any kind of a guard dog. However, he has a huge, loud, very intimidating bark that does make some people back slowly away from the front door :D
He's never been wary with strangers, and our last boxer wasn't either. He always wants to greet everyone, but there are some people he seems to really, really like.
Our first boxer hardly ever barked. He only barked when he was outside playing with the dogs next door and when he saw a UPS delivery truck. Zack barks a little more than that, but still not a lot.