I've been an avid reader for 67 of my 71 years, and for a long time I've been adamant that a book must be good to make it a worthwhile read. It needn't be great--not every book will be so masterful that I'm sorry to reach the end--yet, if it's mediocre I won't waste my time on it.
The mediocrity of a book is nearly always evident in the first few pages--sometimes, the first few paragraphs. If it can't grab my attention and hold it (and I read with an editor's eye--can't help it), I won't bother going further.
Not long ago I began reading a book by an author I'd long enjoyed reading, but this time he wrote using a gimmick (beginning the story at the end and writing backward, chapter by chapter).
Because I'd always liked this writer's previous novels, I spent more time than I normally would on what became unwieldy, uncomfortable and downright unpleasant trying to hold the various threads together in my memory and keep up with the 'backward' plot. I won't call it mediocre writing because one couldn't use that technique without cleverness and a great deal of organization. It was, however, a huge disappointment. I gave up after a few chapters and will be reluctant to read any more of his work unless reviews show he doesn't try that stunt again.
I'm a fast reader, so there are few books I can't or don't finish in one sitting. They must engage my imagination, and the writing must be good enough that my editorial eye doesn't cause me to become overly distracted. When a book captures my imagination and holds it, that, to me, is the joy of reading.