Lycos may work well if precision is important, but does not allow phrase searching. Personal preference plays a role as well. Search forms can be simple and non-transparent, or complex but customizable. Excite offers ranked listings based on weighted comparisons of terms in documents. OpenText has a flexible search form and is fast.
Output varies, as well.
More sophisticated search tools produce detailed entries containing abstracts, outlines, and frequent words used. However, these abstracts are machine generated, and shouldn't be taken as measures of quality. Z39.50 is a standard communications protocol which allows the user to search the online catalogs of several libraries, using a common interface. For instance, a person could perform a search on Madcat, then perform another search on the University of Minnesota's OPAC, without learning new commands. There are also links to the Z39.50 gateways of several universities, including Madison.