I use the following subjective ratings to classify my non-fiction consumption. In the perfect world, all my reading would fall into ratings 1 and 2 (with the odd 3 now and then, as the mood takes me).
-- Francis Bacon
Blew my mind. (Or just a "classic" text that needs to be read.)
2: great stuff
Has depth: good ideas, well put together. Changed my world-view
3: worth reading
A good account, worth the time I spent on it. But nothing necessarily earth-shattering or paradigm-shifting.
4: passes the time / mind candy
I read it, but it did nothing much for me: maybe I thought its style was turgid, or its content was shallow, or incomprehensible, or wrong! Or I just wasn't interested in the subject.
5: waste of time
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, probably skimming bits: if I'd been interrupted, I might not have picked it up again.)
6: unfinishableThis book fills a well-needed gap in the literature.
I couldn't finish it because it was so bad, or so boring, or so wrong, or whatever. Or I just found it too incomprehensible to bother with. (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.)
Long books, when read, are usually overpraised, because the reader wishes to convince others and himself that he has not wasted his time.
-- E. M. Forster