I currently have over 5000 books in my library, and last Christmas I donated over 1500 to different organizations. I have the space for my books, and have reread all of them at some point, and will continue to over the years.
I donate the books that I didn't really enjoy.
I say if you have the room, and like the books keep them, otherwise, donate - but be careful. My library wouldn't take the and I had to do some research to find organizations that would take them.