Like anything else, there is always hype surrounding products. I do not recall any seller ever posting a negative testimonial. The fact is even the worst, most ineffective product can have rave reviews from some customers.
On the other hand, it does not mean that a product does not work when it does not have a lot of testimonials or just does not publish them.
A better way to evaluate is by using statistical data, at least when it is available.
Also, another thing that sellers rarely do is to keep you well informed about possible negative effects or dangerous substances in their products.
Another thing that you can do is to read back through this forum and look especially for posts that are marked SUCCESS or EDUCATIONAL.