I got rid of maybe 5 bags a few months ago. I'm really not a book keeper. Other than maybe 20 or so favorites, if it were only me, I'd get rid of all of the rest. DH isn't ready to part with all of his yet.
I don't get the need to keep books. I now get almost everything I read from the interwebz or library. It's such a beautiful system-read for free and return. I do have reference books in my office, but even then I don't use most, and could get rid of all but maybe 20 or so. But, then I'd have a whole wall of near empty bookcases, and as a professor, I can't really have that, now can I.
I have to say there's a real snootiness among my colleagues regarding the possession of books. I personally just don't get it. Why pride oneself in being inefficient? Maybe in other fields there are old texts that one needs personal copies of b/c of how hard they are to get. But in the sciences, journal articles are where it's at, and even then, I keep most copies electronically.