Call your lawyer.

User generated content, as opposed to editorial/reviews by site staff, can cause problems. You'll have to check with a lawyer in your jurisdiction for legal advice. That said, if you have a TOS that indicates you do not edit/moderate UGC and the opinions are those of the poster, not the site, you might have some protection.

An opinion is an opinion surely and we are free to express that opinion unless the law states otherwise.

I've had a couple of business go hopping mad about some of my reviews and when they write with the threat of legal action I've told them to go ahead with it. No one has ever got back to me. Presumably they checked with a lawyer (at their expense) and were told that there is no law against expressing an opinion that contains no inaccurate facts.

The only extra security I take against legal action is that I photograph anything I factually criticise. So, if I state in an article that the layout at establishment A is drab and boring, no photo neccessary. But if I say that establishment B has eleectrical wires hanging from the ceiling then I take a photo of that. I also publish the photo in the review so, up front, they know that the wires really are hanging from the ceiling, and I have the proof.

I like what is printed in the footer of every page of WebmasterWorld:

All trademarks and copyrights held by respective owners. Member comments are owned by the poster.

If you publish user generated content and it turns out the content violates laws, in most countries you are liable as soon as you get notice of the violation.

So it usually depends if the content violates any laws or not. Normally you are fine with 3d party reviews as long as they are not defamatory or slanderous.

All trademarks and copyrights held by respective owners. Member comments are owned by the poster.

Comments are owned by the poster, however, there is case law in the United States where the medium used to convey the comments can be held liable (complicit or culpable) for the presence of the content, even if it is the poster's opinion. While this is often applied to print media in regards to "letters to the editor" which are published, that is not to say it cannot be applied to any site that allows user comments. Remember this, anyone can be sued for anything. To believe otherwise is, simply and bluntly, stupid.

Freedom of speech is guaranteed by the Constitution. Freedom to post something on a website with impunity, e.g. the website owner's not removing it or you not being sued, is not. Think cyber bullying: they're opinions too, but they're also actionable.

Bottom line advice, CYA!

I have had this happen to me a few times. My usual response is to send a reply asking for evidence that the negative review is fraudulent or intended to harm business rather than being an honest review.

90% of the time I will hear nothing back, when I do hear something back it is often blather that I can respond to with a few sentences. Only once have I taken a review down when I received a complaint that the review was posted by a competing business. I dug a little deeper and emailed the original poster, who was very petulant about the insinuation, until I looked deeper and found that particular guy had posted all negative reviews about that particular business, but was commenting/reviewing another business none-stop.