I'll chime in, as a dog owner myself, even though much of this has already been said:
1) Only get a dog if it is appropriate for your situation, and only if you are willing to give that dog care and attention.
2) For most people the goal in having a "watch dog" should be limited to having an alert animal who will notify you when trouble is coming.
3) For the most part, true "attack" dogs are either chosen from good breed stock, and then highly trained (thus, expensive), or are dangerous and unsocialized animals. You will rarely run across a well-socialized pet dog that you can expect to reliably bite the bad guy, while still behaving with the good guys.
4) Having a dog that only provides a visual and auditory deterent to criminals isn't necessarily a bad thing. They don't need to attack to be useful, and they should be regarded as a part of the family, rather than being chained in the alley at all times.
5) If your dog fails to perform as you expected against a criminal, don't blame the dog. Just because you believe that your dog is a highly skilled burglar-eating machine, doesn't mean that the dog believes it should be taking on this role. As such, only get a dog if you want a dog!
6) If you want a dog that barks at every possible threat, just remember that you are going to end up with a dog that barks at darn near everything!
For my part of it, I have an 80lb German Shepherd/Akita mix at home. She's a great dog, and loves everyone. I can take her to the park, she can play with children, and she goes on hikes with me. She does a good job of alerting me to something approaching the house, particularly at night, but doesn't bark much otherwise (sometimes her "alerting" consists only of whimpering at the door because she thinks a "friend" is approaching). My dog looks bold, but I wouldn't expect her to attack a criminal... It isn't to say that there isn't the possibility that she would if her home was threatened, but I certainly wouldn't count on this sort of chance behavior.