Having a good understanding of defamation law is the first thing you need for writing reviews. Opinion is protected. False statements of fact are not. However, the plaintiff has the burden. How would they prove long after the fact the condition of your hair, or anything else. I would not be seriously worried.
The websites are fully protected under the CDA. The actual posters are not protected there or writing on their own sites. Just ask a few bloggers getting caught up in court in Florida, or Crystal Cox out in Colorado.
There are practical issues. If you are trashing a financial services company with lots of money and lots to lose from bad press, then you can pretty much expect to get hauled into court. I rather doubt a hair salon or any small business is going to have the resources or want to risk lots of additional publicity over the issue. Most states you can't do defamation in small claims, so it is a major deal to bring a civil action. The business, if a corp. or LLC, is going to have to hire and pay an attorney. Due to tort reform, the winner generally does not get attorney fees awarded in a defamation case, so the jury award (assuming the defendant wins) might well be smaller than the legal fees.
Lawsuits against ripoffreport are a waste of time and money. They are deadly serious about fighting them and they always win. Then they sue you back for malicious prosecution and they do it often in Arizona where you will be hard pressed to defend yourself. However, lawsuits against posters have been successful.