I have not personally used it (though did at one point question giving it just as a preventative), but there is a woman on another dog forum I go on who SWEARS by it. She has a dog that had a quickly growing lump and giving the essiac tea caused it to start decreasing in size, until eventually it was completely gone.
My opinion is that it won't hurt, so I'd definitely try it.
Juno CGC: 3/31/09 "Reverse" Brindle, docked & floppy
Lucy: 2/1/06 Bichon/Poodle Mix
Happy: 1/2001, Rescued Border Collie Mix
RIP Molly, January 1997-April 8, 2009