It all depends on the type of research you are doing. Coverage is predominantly medical, scientific and technical. It is not the best source for social science or humanities topics.
Google Scholar should never be the only database you search, rather it should supplement more comprehensive and authoritative databases provided by Gumberg Library.
The University of Tennessee Libraries created several helpful scenarios to figure out when you should/should not use Google Scholar for research:
Not necessarily. You will still need to evaluate what you find because Google Scholar includes material that may not be appropriate for your research. Some of these items include pre-edited articles and reports, as well as theses that may not be as scholarly as other resources. You may also find errors in citation information. Remember, not all scholarly journals are indexed in Google. Many important journals are not included, so you should not base all of your research on what you find in Google scholar. You may be missing some very important information. Google scholar does not cover material written pre-1990 as well as subscription databases do.
After you conduct a search in Google Scholar, you will see some references that include a link which reads "Cited by" followed by a number. This number tells you how many authors have referred/cited this particular paper to their own work. Clicking this link will take you to a new result list of these citing articles. This can be especially helpful when searching for sources, as one particularly useful article will link via "Cited by" to additional articles on the same topic. And if you are an author, this link will provide a listing of others who have used your work, which can help gauge the scholarly impact of your citations.
Note: Google Scholar only includes articles that are indexed within its database, which is a much smaller portion of full-text articles than can be found in Gumberg Library's subscribed databases.