I've had a colour-coordinated bookself for the past two years now and don't think I'll ever change the way I organise it. Whenever anyone asks me how I ever find anything, I question why I wouldn't be able to find books - if you've had a certain organisation for two years, of course you'll be used to it by now. I know my books by colour. If someone comes to my house and I want to show them a book, or they want to borrow a book - I automatically think of what colour it is, and then search for it within that realm.
The rainbow of colours is aesthetically pleasing, it brightens my room considerably, and I can't imagine ever shelving my books by series/author, because then what do I do if I just have a whole pile of standalones? the colours would look like a kaleidascope threw up on my shelf.
HOW DO YOU CATEGORISE YOUR BOOKS? BY GENRE, AUTHOR, COLOUR OR SERIES? OR DO YOU NOT CATEGORISE THEM AT ALL?