Today's halachos. (You can continue talking about yesterdays.)
In order to post a review online (which is definitely l'toeles), you still must fulfill the rules of toeles. Here are some potential issues to keep in mind:
1. One’s words will cause damage to the person (which is included in the issur of Loshon Hora, and also the issur of Gorem Nezek Lechatchila), so the damage must be halachically warranted. If it is not (where the rules of toeles have not been accounted for), it is forbidden. [It is forbidden, at least lechatchila, even to cause damage to Akum.]
2. One must be entirely factual, and not exaggerate even slightly. Thus, one must also avoid using opinionated negative words like terrible or awful.
3. One must be motivated by the need to help others avoid the pain that one experienced, and not motivated by a desire for revenge.
4. One must be absolutely certain that the assessment of the product or service is accurate. Believing that the product line as a whole is bad just because yours was defective is, to say the least, unscientific. If you have reason to believe that you will be one of the only ones rating the product, it would not be a good idea. Moreover, even if there are many negative reviews, one must consider the possibility that only those with bad experiences are posting reviews. (I would hope that any review readers consider this possibility, too!)
5. Similarly, one must consider the possibility that people will see the negative feedback of a product to be reflected on the merchant. As with most cases of toeles, one must avoid causing more damage to the merchant than is warranted by halacha. Thus, one must be certain to state that the issue pertains to the product, and not to the merchant (if this is indeed the case).
6. Regarding rating the product (such as giving it one or two stars out of five), one can rate it the amount that it deserves – but be sure to back up that rating with facts. I would hope that consumers understand that the rating is completely arbitrary (there are no rules that govern the rating, after all), and they would read the basis of the rating before coming to any conclusions.
Teshuva almost word for word from Rav Berkovits himself (I changed some "He"s to "one"s), this time. Printed in the back of Dovid Jaffe's sefer on Hilchos Loshon Hora, "What Can I Say Today?"