Well, I am in a posting mood right now, so here goes.
an opinion on a domain's worth can help you to adjust your asking price, if that opinion contains well thought out arguments.
If you pay for an opinion, I think there is a conflict of interest, as the person or company is likely to over value the domain, to encourage you to buy more opinions.
I think the most helpful and honest opinions can be obtained when the people giving the opinions do not know what other people's opinions are. That will cause them to do a bit of thinking on their own. As opinions are listed, many people who add to the opinions, will be affected by the opinions already posted. Some people might be reluctant to go against the general trend, some people may feel like countering the general trend, to gain attention, ect. I think most people do adjust their opinions based on what others say, as the psychology of the crowd usually has an effect one way or the other.
I think if you can get several independent opinions, it can help you decide how to price your domain.
In the end, you can have several valid and reasonable opinions, but as we all know, a domain might sell for much more or much less than any well thought out opinion.