Google Scholar offers a simple user interface and is open to the public, unlike many specialized research databases. Yet some scholars act as if Google Scholar is not a "serious" research tool. Is it reliable? And when should I use it?
In my experience, Google Scholar is not a replacement for other library research databases (those available through EBSCOhost, Web of Science, etc.). These have particular strengths, and are essential tools. Google Scholar is useful not only as a supplement to these, but also because it has its own particular strengths. One good use of Google Scholar is that it incorporats a ranking algorithm that can be helpful when you are learning a new field: for example, if you search for "economic sociology" in Google Scholar, the list returns pages of important and central texts to the subfield, so you can quickly get a sense of what is important. However, if you are already familiar with a subfield, this ranking algorithm can return pages of hits that are irrelevant to what you might be looking for. Another good use of Google Scholar is if you are doing more historical research: through agreements with University Libraries, Google has incorporated lots of journals dating from the early 20th century (and older) and applied OCR technology to make these fully searchable--many of which are not fully indexed (or indexed at all) in more traditional databases which have a more contemporary coverage.
So my short answer is that Google Scholar is an important tool, but should not be the only search tool you use.