To Review or Not to Review: 1 and 2 star books

I've been having a bit of a conundrum when it comes to books that I don't really like due to the quality of writing, plot, characters, etc.  Do you write a bad review?  Sometimes I feel like it's a waste of my time.  At the same time, though, shouldn't I be honest and cover those books that weren't well done--especially if I spent the time to finish it.  I go back and forth.  

If I don't finish a book, I won't review it.  Not formally.  I may post something brief about why I didn't finish it, but I don't feel like I can really write an honest review if I don't finish it.

What do you think?  If you are giving a book 1 or 2 stars, then do you write up and post a review for that book?  Or is it enough to just not promote it?  

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