depends on where your doing it. But I wouldn't do that unless you REALLY have good reason for it. You will have a lawsuit in your mailbox faster then you ever thought.
Honestly, I would consult a lawyer on it. I believe you have the right to voice your opinion. I believe you could also create a 'sounding board' for others, such as consumerist.com or a complaint website about the company, at a time when there was a bestbuysucks.com (it's being parked now). The safest thing would be to talk to a lawyer and/or complain to the Better Business Bureau and/or attorney general. It depends upon your country, city, and state you live in, but you may have similar options.
Becareful you dont go to far and get hit for slander. Of course you can do it but it is extremely against business ethics. Think of beer commercials, bud light always calls out miller light and vice versa. But they dont go as far as saying the beer can potentially kill you or something. Just stating facts. Believe me ive thought about it. You can always post pricing of another company to show you are cheaper.
Honestly, if you can get the facts right, together with evidence to support your statement, then you should be fine. Whatever you do on an anti-"whatever" campaign, it's crucial to ensure your evidence supports the facts.