Interesting question this one - and I think that there is no clear-cut answer.
The value of a forum is directly related to the quality and integrity of the 'posters'. As this cannot be tabulated it is not possible to answer your question. A good guide to their integrity is - does this poster who has just posted a 'rave review' or a 'trashing review' post under a nickname with no contact details, no website and no way of knowing who they are. Is it one of their first posts? Have they a history of commenting a variety of manufacturer's products?
For example - over the last two years someone joined this forum using a similar user name to me and proceeded to consistently trash Gardiner products and praising a newly launched direct competitor - I knew already for a fact which company this person represented and when CIU Admin tried to contact this user - amazingly the email he had given was false!
This highlights the issue. As far as my company is concerned we have never posted under a pseudonym, we have never asked clients to post praising our products in order to receive favour or products. All of our clients that choose to post are genuine product buying clients who have to part with their money for our products. These clients/posters are also quick to tell us if they do not like something publicly or privately!
What I have learned from these forums is the value of an honest and experienced forum user. When they post something about a product I always listen, if they do not like something we make I listen even more carefully and if they come up with an idea to improve something I pay attention.
On all of the UK forums there are some very experienced and canny window cleaners. My advice is to look back at key product posts and compare certain posters answers with your own experience. You will start to build a pattern as to who to listen to and who to ignore. I for one value the collective experience of the many of the posters on this and other forums.