I don't post much, but I thought I would throw my $.02 in.
I grew up with a Boxer from the age of 8 to 16 and I think they are great guard dogs. My parents wanted a Dog to protect us kids while we ran around our 10 acres and to keep critters out of the garden. A friend raised pure bred Boxers and recommended them, so we got a 6 month old male pup and named him Sam.
Here's my opinion on them;
They are loyal, gentle with family, fearless defenders, and intimidating to strangers. They are not known for being highly intelligent, Sam was smart enough, yet he was stupid on somethings. In the summer time if we didn't lock him up in the evening, he would almost always find a skunk and get sprayed. He also hated opossums, every since an incident one night when he thought one threatened us, he killed them.
He was generally good with people, yet he disliked anyone that walked onto our property, and would usually "tree" visitors if one of us wasn't around to tell him it was ok. if someone drove in, they were treated better and usually only got barked at until one of us told him it was ok. though most people wouldn't get out of their cars until we were there. One time a US Marshal that came to our place by mistake.. and wouldn't open his door till we had locked Sam up.
Sam would take on any thing he considered a threat to us, he weighed 90 pounds and would take on dogs that weighed 140. namely our neighbors aggressive mastiff mix. In short, I think Boxers are great dogs for family's with kids, but don't get one if you want a dog with the brains of a Border Collie. If you want a loyal, loving, fearless defender, strongly consider a Boxer.