Briefly, the advantage of a Meta-Search engine is that it brings you results from different engines. The downside is that you can't use some advanced syntax since the syntax on different search engines is different.
Here are meta-search engines that were brought up in my Boolean month-long workshop. AllTheInternet.com; Clusty.com; DogPile.com; IxQuick.com; Juju.com; KartOO.com; KillerInfo.com; Mamma.com; MetaCrawler.com; Search.com; SurfWax.comn; Visismo.com; WeCrawler.com; and Zuula.com.
I like searchboth.com because you are able to use advanced syntax. Of course the two engines you chose have to be compatible. So I find myself using yahoo and google.