Over at slashdot there's a very lively discussion of how Google scholar shapes up against other academic search engines, free and subscription.
The starting point is this review by Thomson Gale corporation, which concludes that other free services and some subscription services (by coincidence including two Gale products) do a better job. Google is popular because of a combination of hype and "laymen's ecstasy," which must be either a fungal infection or a very attractive controlled substance. If you can't guess my opinion of the review yet, take a look at this quote:
The problem may be that Google developers have been working smart and hard to make heavy duty software excavators that dig up useful data from unstructured masses of data from zillions of Web pages. They were great for discovering the Web landfill, but not for digging scholarly archives — just as heavy duty excavators are inappropriate for archeological digging of Mayan tombs instead of tiny pick axes, chisels, shovels, trowels and brushes to extract, clean, bag and label the finds.
This begs the question of how the preciousss tombses are located in the first place. Has this guy never seen a field survey? We're talking about search, aren't we?? Moreover, this tone, which is used throughout the review, comes off like the anal-retentive chef in a new calling, and leaves me seriously wanting to ignore their products.
This is the kind of topic that slashdot does best, so it's worth reading the whole thread. I think the main meta-conclusion is that there is a LOT of interest in academic search-- so, if nothing else, Google is onto something. In terms of the specific debate, I use google scholar alongside the main free biology database ( PubMed ) and usually find what I'm looking for. The main limit on both of these is that electronic versions of anything published before about 1996 are nearly absent.