I train my male in IPO, and he is perfectly stable and safe around kids, people and dogs. Although it depends on the dog's base temperament and nerves, this is the case for most schutzhund trained dogs.
Schutzhund tends to bring out the worst and best in a dog, it will not make them "meaner" unless they are already "mean" if that makes sense. Although "mean" isn't a good term to pin on a dog.
Chris Wild's article is my favourite:
(Schutzhund Training, by Wildhaus Kennels )
I have a section of articles on my website for IPO info, if it would interest you:
Articles - German Shepherd Guide