When it comes to Hotels, I find that tripadvisor has become totally useless in some parts of the world. Many hotels have either extremely negative reviews or extremely positive reviews. Often, when you delve a little deeper into those reviewers, patterns emerge and you are left questioning the motivation of the reviewer. When it comes to restaurants, the ratings are even more unreliable.
On the other hand, sites that incentivize buyers to come back and review products tend to be reliable (unless the site's management start interfering).
IMHO, buyers need to be verified in order to have accurate user ratings.