Interesting to note that this is where TripAdvisor initially started. It turned out its really hard to scale this 1) globally and 2) efficiently and thats where UGC came into play.

Question on the trust factor: Why should a user trust "professional" reviews over the collective opinion of the crowds? Professionals can never experience everything, so will be working from a smaller knowledge base to start and are probably more corruptible than 150 MM people or wherever TripAdvisor is these days.