Remember that, although the main goal is finding low prices, that there are other things to take into consideration when judging the usefulness of a site. For example, you want to make sure that, at least for popular and widely-distributed products, you get a good number of search returns, and not just a few. This will help insure that the site is using a good formula by which to search for the best prices, and not just focusing on a few retailers who happen to be partners with them.
Also, you want to make sure that the searching capabilities provide accurate results. This means, for example, if you are looking specifically for a camera, that there are ways to conduct your search that doesn’t bring back price results for accessories, such as camera cases. Being able to filter out unwanted items, while focusing specifically on what you need, is important to getting good results.
Another way to judge shopping comparison sites is to read what other people think. By reading consumer reviews on the various sites, you can get a good idea of whether people are generally happy with the site’s ability to locate the best price.