I have read many great books; most of them aren't great with their endings. That does not degrade my overall impression of those books still. Maybe it is a little bit disappointing to discover that the closure of the book with which you have been completely absorbed over a particular span of time does not live up to your anticipation. Although such is the case with me, the disappointment is ephemeral usually. As time passes, I would reminisce on the great moments and ponder the depth of the book.
A great ending, though, can lift my mind to a higher plane for a while (few minutes). Upon returning to my senses, I would ponder the ending, reread it, and
take a walk outside, only to come back and read the ending again -word by word.
The ending to The Great Gatsby is such an ending.