So, in answering your question. Of course I review my friends piece. I'm already prejudiced by the fact he is my friend. If the piece in question is absolute dog shit, I have no problem to tell the friend that too. It will save him money in the long run because if it's that bad than there is no money to be made anyway.
So you do your friend a favor and write a review.

Good reviews help you with sales, bad reviews help you with product development. If enough people told me that a speaker I make with a crappy part doesnt sound so good, do you think I'm going to keep building it with out making a design change just because "i" like it? being self centered isn't a good way to manage a company; so of course i make a change. If sales are good for something then you figure out how to push them out until there is no more market. then wait a few years, make cosmetic change and introduce a new product. It's already been tested as successful.

The loyalty is with your friends. If they remain your friends while you converse on common ground and not loose sight of your morals than the friendship is wholesome and working. If you don't speak on common ground and jeprodize your morals then that person is not a friend. you must move on.

why do you ask grasshopper?