I think it's best used for amusement.
When you request a translation, the text is passed to a web service run by
WorldLingo. Like all machine translations currently available, its results
are... ummm... of variable quality. You might use it to get the sense of a
passage that a native speaker of the "other" language wrote, but you
wouldn't want it to translate your text for something you're sending out,
especially if it's business-related or sensitive.
Let's take a simple example. The English sentence "I do not want a fish" is
adequately translated by the service to the Spanish sentence "No deseo un
pescado." Now take that Spanish sentence and ask to translate it back to
English. The result is "Nondesire a fish." The idea is there, but I wouldn't
call it a good translation. (By the way, you'll get the same result from
http://babelfish.altavista.com.)