If you would be reading a classic, a book-everyone-should-have-read-in-their-lifetime, and halfway through you'd find yourself hating it, or worse, being bored with it, would you still continue, or move on to something else (and possibly better)?
I’d finish the book. Two reasons. (1) Sometimes books surprise you midway through. For example, I hated the first half of Jonathan Franzen’s Freedom but enjoyed the second half enough to end with an overall good impression of the whole. (2) On principal, I’m horribly thorough. I hate discarding projects halfway to completion. For some things, you can get to the point where there ceases to be an end in sight - like waiting outside a nightclub hoping to get let in - but I don’t include books in that nightmarish category. The sunk cost of finishing a book is so minimal that I have no problem forfeiting my time and effort in reading the whole thing. Who knows, I might just learn something.