The majority of books with underlining that I come across are library books and the excessive underlining, exclamation points and asterisks are distracting and annoying.
However, if someone writes intelligent comments in the margins (esp. if there’s a theme), that’s often very interesting.
When I was young I used to underline and write comments in books. I still own one of those books. It’s a Kierkegaard anthology – of all things! – and reading those comments now is surprising. It’s hard to believe it was me who had such good insights! Nowadays, I’ll read a page or two of a book like that and am sound asleep by page 3!