People are more likely to speak about a poor review than a good one. There is some statistic such as poor experiences are more likely to get around than a good review. There is a reason for the discrepancy in amount of positive/negative views mostly because they remember a poor experience more than a positive one (http://www.nytimes.com/2012/03/24/your-money/why-people-remember-negative-events-more-than-positive-ones.html?pagewanted=all&_r=0). Also unless there is an incentive, I feel like not many people I know review things for free. For example, I am more likely to fill out a survey to be entered in an iPad drawing than doing a survey for nothing in exchange.
There are also more efficient ways to view information about keywords, such as using keyword spy, or keyword kung fu. They can tell you information about what sites are currently using which keywords and the competitors’ budget, not just volume of a certain keyword. These insights might allow you to add to your list of keywords that were not previously thought of for your online marketing campaign.