I can't say I do it much, I very much so enjoy doing so from time to time, but I always have a veritable village of unread books lying and towering about in here, so it ends up not happening much.
There was a time when I thought re-reading seemed silly, partly because of a certain materialist sense that I'd rather spend that time on reading something new, and, in a way, "up the total" for some self-gratifying sense of completion over retreading. This has disappeared over the years though, and I find myself more and more often looking forward to re-reading books and even putting some aside in a pile of stuff to get to again.
I've been searching my face off recently to find my copy of "The Great Gatsby", as I've had a strong urge to re-read it. Alas, I can't find it anywhere. Curses!
Most re-read author is Terry Pratchett, oddly enough. Probably because they're generally light reads that I can breeze quickly through and feel rather comfortable.
Saddest re-read: Douglas Adams' "Long dark tea-time of the soul". Mainly because I didn't realize I was re-reading it until I was two pages from the end! Didn't like it much either, which is the only explanation I can find as to how I managed to so totally block out the poor thing. Or my original read was at such a young age that I still had trouble to figure out the English text - possible as the re-read was at age 18, and it must have been quite a few years since I'd first read it.
Odd that they're both popular British SF-oriented comedy authors. It's not exactly a common descriptor of authors I read, though it's true that pretty much all the comedy novels I read are by brits. Wodehouse stands out as the clear favorite, though Jerome K Jerome's "Three men in a boat - To say nothing of the dog!" certainly deserves a mention.
At age 13 or so I first read "Ender's Game" which resulted in a small Orson Scott Card fan-period where I read as many books by him as I could get my hands on - which turned out to not necessarily be a great idea. I can clearly remember making a vow with myself to reread it in ten years.
11 years on, and it's not happened. I DID read "Ender's Shadow" recently though, which isn't too far off.
Short stories, on the other hand, I find myself often rereading, which has probably been some of the reason I've lightened up on the idea of the concept in the first place. It's tons of fun to reread a story several times and take notice to difference facets every time, trying to focus on different interpretations to see how they'll fit etc.
I don't think "knowing what'll happen" is at all a problem for me, as I also find that I often see movies several times over as well. But I suppose it might also depend on what you're reading. Detective novels etc might not be as much fun when you know where it's all heading, unless you're particularly interested in seeing how exactly the author drops hints and winds the threads.
In ten years I'll probably have given up on new books and just sit in a corner reading and re-reading a ratty paperback copy of "The sound and the fury" while mumbling to myself and picking at the fleas in my beard.