Like many of my colleagues in the library-world, I’m often asked how good the likes of Google Scholar, Scirus and Science Research really are. Some people claim that they are serious competition for Web of Science, others that they can’t hold a candle to the subscription scientific literature databases.
Here’s an interesting and concise comparison between the two heavy-weights which may help: Google Scholar Vs Web of Science: McDonalds Vs a Gourmet restaurant?! Note that Scholar is often used in tandem with searching Web of Science. Personally, I find Scholar as a useful starting point – a place to try out and refine my search strategy.
Of course many can’t afford to access subscription databases, and so “get by” on the free sources. I don’t think the free sources should be dismissed with disdain. The main concern is usually the breadth of their coverage. While my main gripe is usually with the search interfaces of the free sites, they are too simplistic and, certainly in the case of Scirus, don’t always parse your commands in the way that you intended! But they will be developed and improved.