I use the following subjective ratings to classify my Science Fiction consumption, including the SF books I read. In the perfect world, all my reading would fall into ratings 1, 2 and high 3s.
An SF "classic", that should be read by everyone. Some classics are just excellent SF; others have contributed to the development of the genre and need to be experienced to get a better appreciation of the genre as a whole.
Good SF, well put together, with some definite depth to the plot, characters, or ideas. Definitely re-readable.
-- Lucy Kemnitzer, rec.arts.sf.written,
A good enjoyable story, worth the time spent reading. Some may not be particularly deep -- or maybe not particularly SF-nal (just placing a plot on another planet, or in another time, doesn't make it SF). Some may be re-readable.
It passed the time well enough, but I probably won't be reading it again.
I read it, but wouldn't have if I'd known... It might have been saved from unfinishable only because I read it in a single sitting: if I'd been interrupted, I might not have picked it up again.
I couldn't finish it because it was so bad, or boring, or whatever. Or maybe the style just didn't appeal to me. I probably said the Eight Deadly Words: "I Don't Care What Happens to These People" ... or Beings, or Artifacts, or whatever the plot is 'about'.
-- Dorothy J. Heydt, rec.arts.sf.written
I classify more books as unfinishable that might have been just a waste of time a few years ago, as I have less time to read, so tend to be more impatient.
-- Dorothy Parker (of Atlas Shrugged)
I must here give a vote of thanks to Terry Brooks' Sword of Shannara --- my first ever 6-rated book. After slogging through about a third of it, I suddenly realised that, just because I had started reading a book, didn't mean I had to finish it. This crucial insight has saved me much valuable time over the years. Once I have discovered that I have wasted my money buying such a book, I no longer feel the need to waste my even more precious time reading it.
-- William December Starr, rec.arts.sf.written