We've got two Jack Russell Terriers - Oliver (5 years) and Jackie O (4 years). Both were rescue dogs although Jackie O was in pretty good shape coming from a family with a special needs son that didn't do well with her. Oliver, on the other hand, was rescued from an animal hoarder by the SPCA. Poor guy - the first year of his life he was kept along with over 250 other dogs in a series of chicken wire pins on asphalt out in the Texas sun and never let out so he didn't learn how to "be around" humans and was VERY skiddish when we first got him from his foster mom. His front teeth are worn down from trying to pick food off the ground when those crazies threw it into the pins - very rough early life. He wouldn't let us touch him when we first got him, but he's a huge sweetheart now because Jackie O (who's definitely the alpha dog and has all the energy Jack's are known for) actually helped bring him out of his shell. They are inseparable now.
I agree with earlier comments that you have to consider the breed and personality/temperment of your dog when thinking about adding another one, but in our case it's a perfect match. We don't have kids and have always had two dogs - this is the third pair - and it's always gone well.