Ashton Kutcher already has experience in raising children, as he helped in parenting the daughters of his ex-wife Demi Moore. Also, Ashton Kutcher as well as Mila Kunis are in their 30s, an age group that is young enough to handle raising a child but mature enough to handle the difficulties and issues that come with being parents.
I think they have just as much chance as anyone else at being good parents, they both seem like decent people. Neither seem to suffer from a unreasonably inflated ego, and stand a good chance at being able to distinguish between a healthy parent-child relationship and one of these bizarre celebrity-style ones.
Ashton Kutcher gave one of the best speeches I have ever heard at a Kid's Choice award show several years ago. I had my kids watch a youtube clip of it later. Kutcher explained to the kids that his success was because of hard work; and that he never quit a job before he had another one. If he shows that mentality to his child, the kid will be in great shape.
I think Mila and Ashton will be good parents to a degree. They will be able to provide for their child due their high incomes and that makes a huge difference. At the same time I doubt they'll spend a lot of time parenting. They'll hire nannies and help and won't be as big an influence on their children as typical parents are.