Never slag off your competitors - as in never EVER! However bad they are. It's just not good business etiquette and you may find it will backfire on you. Nobody likes to deal with people who are bitching and moaning. Don't get into negative gossip about anyone unless you are happy for it to bite you in the backside at some point.
Recently we put a shout out for a recommendation on social media for a particular business. Shortly after I had a PM touting for the business that included warning me off another business. Now this person may have very valid reasons for warning me off, but, to be honest, it put me off a little. We let him quote anyway and he continued to slag off the competitor whilst he was here! So no prizes for guessing who didn't get my business.