I only have an electronic Brick (oops, I STILL need to go to the bookstore and pick up one) and that thing is littered with highlights marking passages I like. I always reserve the gold color for deaths.
As for my other books... I mark them up. Don't shoot!
It really depends on what book I'm reading. My pretty pink Alice in Wonderland Barnes & Noble leatherbound classic book will never be touched, for instance. Probably not my "designer designed" copy of Wuthering Heights either.
But books like The Book Thief or Catcher in the Rye or Pride and Prejudice or The Perks of Being a Wallflower... I attack those with a highlighter and pen!
I use a different color for each read-through.
So for the first read-through I use a pink pen and highlighter, second green, third blue, on and on. That way I know, "this is what I was thinking the first time I ever read this". Then when I mark near it a second time and read back over that
a third time, I see, "this is how my thoughts changed to this certain event/quote/dialogue now that I know the full plot and what the ending is".
It's cool to see what was going through my head at that point in time!