DeepDyve - searching the deep web?

DeepDyve is a search engine which claims to search the Deep web - the parts of the Internet not generally covered by traditional search engines. It even claims to bring back just in-depth professional content, results "not cluttered by opinion sites and irrelevant content". A number of the results retrieved by the search engine seem to require a login or payment to access the full text. In fact, it looks quite similar to Google Scholar and Scirus - have a look and see what you think.