I can throw a little light on this as a vendor, who spends money with these sites.
The only site I have had negative experience with was ReefCentral. I was actually in discussions with them to setup a sponsorship agreement. Its really expensive minimum $500 a month. Before the agreement was completed a few people had stumbled across my site and were discussing on the boards. I even had a customer post some pictures of Modular LED stuff. All of the posts were removed, and one customer was banned. Their other LED sponsor had an existing agreement and we were not setup yet. I complained about this; that it seemed weird to me people were only permitted to discuss LED if it was related to one of their sponsors. Their reaction was to ban me and our pending agreement was terminated.
As for ReefBuilders. They did a unsolicited review for us and it was not paid for. I had a great experience with them.
http://reefbuilders.com/2011/09/29/m...asy-diy-demon/
I have not had any other problems with any other sites. Most are pretty reasonable, you get a sponsor forum and a banner ad. Its like any advertising, you have to pay for it.
ReefCentral is the only one I came across where it is a total capitalist system were you cannot discuss items if there is a competing sponsor.