Sometimes they produce excellent results. For some searches, however, you will be better off using individual search engines. Metacrawler Multiple searches of Altavista, Excite, Infoseek, Lycos, Webcrawler and Yahoo. Good, and fast. Inference Find Multiple searches of Altavista, Excite, Infoseek, Lycos, Webcrawler and Yahoo.
Good feature is the clustering of sites into categories.
Not a search engine as such, but a super little FREE program that sits on your desk top and zooms off to search the main search engines for you.