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I bought the expired domain of a competitor a year or so ago and redirected it to one of our main domains and it's done us fairly well. Unfortunately they left quite a few unhappy customers so there has been a bit of PR firefighting to do, but overall it's positive. If I were going to do it again I would have done a lot more research, I was just sloppy. MaryAnneG is absolutely right, you need to do a lot of research before you buy one, there are plenty of people out there artifically making their domains look much better than they are.