I don't think I made this point clear enough before, so I will emphasize it here.
There is an umdena that a restaurant owner wants people to write - and read! - honestly written reviews. This umdena applies provided there is both positive and negative (or just positive is fine too) and that the review is accurate. Such a review can be read by anybody at any time, and it is okay to have negative aspects in the review regardless. Rabbi Berkovits has said this explicitly. He even commented that allowing only the positive means that the reviews are worthless, as has also been noted here. Allowing the negative emphasizes the positive, making the reader think that the writer is writing lesheim shomayim instead of with an agenda.
One example of this heter of his that was published (again, back of Dovid Jaffe's "What Can I Say, Today?"):
Q. May one rely le’ma’se on speaking loshon hora if the subject gave reshus? Rav Berkovits’ teshuvos about book reviews and public praise indicate that he subscribes to this kula even as a psak for the hamon am. But in Chofetz Chaim: A lesson a day, he writes that the Chofetz Chaim did not want to publicize this kula because it may be taken too far.
A. One should differentiate between personal permission, which one should not rely on, and a situation where it is assumed that one gave permission (an honoree, a book writer, etc.) where one can rely on the heter.