Since Monday, Westlaw Campus has offered SEC filings back to 2003. They can be searched in the News and Business party of Westlaw Campus. (In the past year Westlaw Campus has offered News and Business information, in addition to legal cases, statutes, etc. Although I haven't made a great study of this change, it is likely do to the fact that West is now part of Thomson, which owns many business information sources.)
So why would you want to search SEC filings on Westlaw Campus rather than Edgar Online I-Metrix, Mergent, Thomson Research, or Hoover's? An advantage is you can easily select specific exhibits to search, such as subsidiaries, or you can combine a keyword search of Hoover's company information and SEC filings, for example CEOs who have been hired in the past year. You can keep track of your research using their research tracker. You can search by CUSIP number, or SEC filing number, which one is unlikely to know.
So why wouldn't you want to search SEC filings on Westlaw Campus? First, we have a limited number of simultaneous users so other databases, including the SEC's free site might be preferred. Also, it seems to be running slow. The filings are available as word documents, so I don't know if they have been rekeyed from original filings or if the documents have been copied from the SEC filings. I looked at several filings, and found that throughout one filing the possessive demonstrated by 's had been transformed into R17;, which was rather distracting, and might cause some doubt on the accuracy of the other information.
Although you can search by SIC Code, you can't search by the NAICS Codes, which have been used for almost 10 years now.
So, you may want to give it a try and keep it in mind.