Personally, I don't much mind underlining. The only time it irritated me was when I picked up a second hand copy that clearly somebody was assigned in some sort of class, where they underlined a full three quarters of the sentence of every paragraph for a hundred pages. Surely that's not the best way to note all of that information. It's actually kind of fun seeing what other people thought was really important, especially if it seems trivial to me. It's like somebody came through time to point to one passage and say "Hey, look! Look here!" One of my second-hand history books has a lot of scrawled notes of disagreement (or contradiction) in it, and it was entertaining to have half a mental argument with the margins now and again. Whoever wrote them was very narrow-minded. I didn't agree with some of the author's interpretations or implications but I hardly think writing "wrong!" in the margins with an arrow pointing to the offending passage is the correct way to discuss things.