When I was a little younger (well, I'm only 31, so it's not like it was ages ago) I reread quite a lot, this one in particular got probably 4 re-reads. My most re-read book is probably Hyperion, by Dan Simmons (followed by HHGG). It's not for every sci-fi lover, but the first part of the Hyperion Chantos (that would be Hyperion and The Fall of Hyperion) are incredibly good books, not only for the science fiction, but also have a thick, interesting plot full of the "I wonder how this turns out" sci-fi.