Such comparisons can come from several different perspectives, those of business, libraries and computer science. From the perspective of a professional user, I generally find my strongest allies in the library community. The following are of interest:

One of the best recent web sites for research purposes is Sprintmail's Web Search Strategies. It provides explanations and interactive exercises that involve the actual use of search engines. These exercises will divide your screen into two frames with the left side providing the instructions and the right side opening and displaying the actual search engine..

"How to Search the Web: A Guide to Search Tools," by Terry A. Gray, an excellent detailed look at the features of AltaVista and nine other major search engines, with a good summary chart.

"Some Search Engines .. and an explanation of how they work", a business-oriented comparison with compact descriptions of each search engine.

"Guide to Meta-Search Engines," by Jian Liu, a reference librarian at the Univ. of Indiana.

"Understanding and Comparing Web Search Tools," a Web bibliography of comparative studies.