My aunt used to raise orphan squirrels and flying squirrels. Cute as babies, then she would introduce them to the outside slowly and they would end up outside permanently in the end being "wild" squirrels. Just like the wildlife rescue thing?

Once they get fully grown, most of the squirrels tend to turn into wild animals and be destructive. I've seen a couple regular squirrels that seemed to be good pets when grown.. but the flying squirrels seemed to be much higher strung and tended to get more wild the older they get.

You might want to stick to sugar gliders, which are similar in build/looks, but tend to be good pets.