My favourite Bible, the first one I was given when I became a believer is underlined and has a lot of notes in it. I find it interesting to read through and see what resonated with me in the past, and it makes me re-focus on those areas of my prayer life. Like others here, I keep another Bible that is unmarked and pristine, but time and again, I seem to defer to the one with the underlining and notes in the margins. I suppose for those who are opposed to this practice, a scribbler can also be used to make notations if they don't want their Bible marked up. It is all down to what feels right for the owner of the Bible. In no way do I feel it is sacrilegious to underline or make notations, and in fact, all of my study Bibles leave spaces for just this purpose. Again, personal taste prevails.