Here's a review on google scholar by Thomson gale . Here's an extract that will give you an idea of what the verdict is:
Google Scholar is muddying the water and makes its hit numbers and citedness scores even less reliable by this mixing. It is as if the bartender put not only tonic in your gin, but also the peanuts. You may ask your bartender to do it again without mixing, but in Google Scholar this is the mandatory way.
and
My usual techniques to get some details about databases don't work here because of the serious limitations of the software and of its disregard for common practices and conventions.
Filed in: google scholar google+scholar limitations study comparison