Like most things, it depends. It depends on the subject, the kids, the computer program, and the teacher in question. Change any of those up and you change the outcome.
Now for some anecdotal evidence. I know with my son, we got a subscription for a home schooling program for use over the summer. He's naturally good at reading, so he's flying through those parts and learning quite a bit. He also seems to be learning more in the social studies units online then he did at school. Math, not so much. He's on track and still doing good work practicing his skills, but it's not as good at teaching new ones. However, even with the math I think that if it was paired with a teacher it would be better than either alone.