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Let me tell you the truth, I have respect for people who stay civil even when a book has wasted their time. It's not the end of the world for them (and for me as well). Still there are some people who feel the need to use mean swear words or curse just to express their feeling which I don't think is necessary. You know, what you write on the internet is the representative of your self, just like your alter ego. Your writing is your reputation, and if you care about your reputation, you might want to rethink your review strategy.
Negative is negative, even without wicked words
I personally won't use swear words, but well to each their own. If you feel the need to stress some thing you don't like, maybe use the milder version like "darn", "freaking", etc. I don't think it lessen the emotional part of the review, but surely it would sound much friendlier.