Dear All Google Scholar is best search engine eve I has used till date, Yes Web of Science is also a good one but it has limitations for e.g. It require subscription, One major drawback of Web of science is on-line first artical is not available while in Google scholar the artical appears within-2 days once it online published. Google Scholar offers citation alerts
For more details about comparison between Google scholar and Web of Science see following link
http://www.harzing.com/pop_gs.htm
Another post
PubMed, Web of Science, or Google Scholar? A behind-the-scenes guide for life scientists. : So which is better: PubMed, Web of Science, or Google Scholar?
http://libguides.lib.msu.edu/pubmedvsgooglescholar
See Nature News on 7th Novemebr
http://www.nature.com/news/google-scholar-pioneer-on-search-engine-s-future-1.16269