I’ve found Google Books very helpful on occasion, but I’ve also noticed that they’re really struggling with quality control. Indeed, I think the first thing I ever looked up on Google Books had a crucial page missing, and was therefore useless to me. Some people write thoughtful articles about the problem. Me, I just take screenshots:
Update: For example, as of this writing, Google’s treatment of the Commentaria in Aristotelem Graeca is a fucking disgrace. A free fucking disgrace, but still.