Internet for Lawyers recently posted a whitepaper in which authors, Carole A. Levitt and Mark Rosch, compare the results generated by the following caselaw citators: Google Scholar, Fastcase, Casemaker, LexisNexis, WestlawNext, and Bloomberg. Registration is required.
The study is somewhat limited in that it only compares the results from one case, but it is still illustrative. The authors found that LexisNexis, WestlawNext, and Bloomberg performed better than their lower price counterparts, Google Scholar, Fastcase, Casemaker. No big surprise there. However, the authors so suggest some workarounds to help expand results.