This does demonstrate the value of third-party "professional" reviews (e.g. Consumer Reports, Zagat) and a big reason why Google bought Zagat -- the trust factor.
You never know if personal reviews on various sites were driven by ulterior motives. Although it's likely that the majority of personal reviews are genuine, you should try not to be swayed by just a few reviews without further research.
Professionals have a reputation, which should reduce bias or influence. If Zagat was known to take bribes, they wouldn't be around for very long.
It may happen, but is less likely to happen than non-professionals. You have a much larger issue with selection bias for reviews, and a much easier way for vendors to bias them. I've been in shops that insist, rather persistently, that I write a good review for their store on their computer, at the shop.
Great points! The issue of selection bias is very real on many review sites. Especially with the volume of reviews being so low for most products and services.
You never know if personal reviews on various sites were driven by ulterior motives. Although it's likely that the majority of personal reviews are genuine, you should try not to be swayed by just a few reviews without further research.