In my case it was a bad idea. I started a business (printing) when I was 20 years old and didnt have the credit I needed to purchase a piece of machinery. My dad said he would co-sign the loan for me for a silent % of the business. Not to long after that, he lost his job and decided to come and work with me. Since he money in the business, and he was 'the dad', he was always right. It totally destroyed our relationship. I ended up leaving the business and haven't talked to him in 10 years. It's a shame but money does strange things to people you think you know.