I had some nasty expereinces with some of the web hosts peddling their stuff here Can I post reviews exposing their inexpereince and shameless deeds in web hosting section?
Just remember that if you're posting to bash a company you did business with through DP, that you should be leaving that review in iTrader or the deal thread and not starting new threads. And as with other areas, don't start threads to accuse people or companies of being scammers. Keep that feedback to the appropriate areas, and stick to the facts when reporting those kinds of situations to others.