I have mixed feelings about Pitbulls...yes, the ones I've met have been very nice dogs, however, like someone said earlier, there is that instinct that is in them and can't be controlled.
I'd be interested to know the circumstances in which the pitbull mauled the sheltie. And you hear so many stories about "nice" pitbulls attacking children.
On the other hand, I met one while working at a local animal shelter. I felt really bad for her because she looked so sad sitting in her kennel. I had just started working there when she came in and I put my hand in her kennel and she gave me a little lick then when it was time for her bordatella vaccine she was so good. I sat next to her petting and distracting her while another person shot it up her nose.
I went and asked my supervisor what would happen to her, whether she would be up for adoption, and he said no, as soon as the week was over she was going to be euthanized. I was horrified that she wasn't even going to be given a chance. But I also didn't know anything about her history, why she was at the shelter. A very sad situation.
My last dog, Sucia, was a lab/shepherd/husky mix. She was I think around 14 when we put her down. We brought her home from the same shelter I worked at, but about 11 years prior.

A couple weeks after we brought her home one of my guy friends came to pick me up and he was wearing a baseball cap. Well, Sucia was NOT going to let him near the house. She kept herself between my dad and I, until my friend took off his hat. My dad yelled to me to tell my friend to take off his hat. Sucia was barking so much we had to yell to each other.

She was a great dog, we put her down in February of this year, and I miss her terribly.