I have a pit bull for the guard dog at my shop. She's just a pup(around 8 mo.) still, but does a good job. She's loose at night in the yard(just a warning: don't reach through the fence, and God help you if you jump it), and tied up during the day. I intentionally keep her away from anyone who is not family or a close friend. Hyper and happy when I get to the shop in the morning, and loves to "attack" my boots. She plays tug of war with my pant legs when she gets bored with trying to chew through the steel toes. I pet and play with her every day, and she is a very happy dog.
She does need a bath, though...
I've had big dogs since I was a kid. Excellent for protection of property when you can't be there. Little dogs always seem to have issues.