What you say must be true - you can't make up a bad experience with a person or company. Otherwise you could be accused of 'defaming' or 'slandering' them (legal terms for saying things about someone or a company that describes them in an untrue negative manner). If a court found that you were defaming someone you could be made to pay money damages in a large amount.

It's a good idea if you are saying negative things about someone or something that you start by saying 'it is my opinion that....'. That way its clear that you are expressing an opinion not a fact. If it seems like you are stating a fact and that statement is later found to be untrue, you could be found to have made a defamatory statement. For example if you said 'Dede's subs uses rotten meat' it sounds like a fact when its probably your opinion. If Dede's sued you and proved that their meat is delivered daily and is actually fresh, you could be responsible for paying damages to Dede's.

Companies, in general, do not like the 'anti' websites and often hire people just to monitor them for defamatory statements. If you decide to contribute to such a site or start one, act carefully.