I just noticed an earlier post about rescue dogs, and thought that I'd just add my thoughts.
We've had two RSPCA dogs, both from the same centre. The first, we still have. He's now fourteen and is a lovely sweet dog, great with everyone. We've never had any issues with him, temperament-wise. However, he was a few months old and had only been in the shelter for a few days when we got him. He had been given up by his owners, as they had moved into accommodation where they couldn't have dogs.
The second dog was a different story. She was another cross breed and was very shy when we got her. The shelter told us that she had been rescued but that she was a lovely dog who was much improved from when she arrived. That was pretty much it. She turned out to have all kinds of behavioural issues. After months and months of perseverance we were at the point of having to return her to the RSPCA. It was only at this point that they saw fit to tell us that she had been physically abused by her first owner, beaten severely with a bicycle chain... We kept her, but it took us four years to get her to a point where she was completely trustworthy with strangers and children. Sadly, we lost her soon afterwards to an illness that our vet said was probably brought on by her nervousness.
In my experience you have to be VERY careful when rehoming a dog and really push to find out as much information as you can, especially if you have children. Dog centres always try to re-home every animal, if at all possible, so it is not always in their best interest to be 'negative' with their information. I wouldn't necessarily advise anyone not to re=home, as there are some many unwanted pets out there, but if you have little experience with dogs I would be extremely cautious.