If the written material is factual and the opinion left out, it's hard to argue on either side. Posts that run along the lines of 'Don't go to company X 'cos they're a bunch of tossers' is a lot different to 'When company X serviced my car they didn't change the oil'.
It can take companies years to get a good reputation, and minutes for the internet to destroy that.
The phrase 'I wouldn't return to company X' should suffice without discussing detail..
