This question is dependent on the researcher's objective. Does the resource have a reputable organization or expert behind it?
As in traditional print resources one must use a method of critical analysis to determine its value.
Also, does the Web site give sources for the information it presents, sources you can look up and verify?
Timeliness is another important factor, especially if you are researching a subject in which knowledge can change rapidly, like health and medicine, business or technology. If so, is the information is current enough for the topic you are researching?
Must you register a name and password before using the site?