I think that is a pretty tough question. Like you said, many people think the interview went great and they weren't accepted or vice versa. First, receiving an interview definitely increases your chances, but then again..it is a chance, not a guarantee.
I think an interview can definitely make you or break you, but it is not the only factor that adcoms look at. I think that time also plays an important role (this is where luck comes into play). Perhaps you get an interview early, and the adcoms is looking for a particular person. Let's say you fit the description and then you are accepted. But if you were given the interview later on in the process, then maybe they are not looking for that type of person anymore and you are rejected.
There is really no way to know how important the interview is for a certain school, until you receive that letter.
As applicants we just have to do as best we can in the interview, in that way we can increase our chances even more for an acceptance.