I do almost all my reading on my phone; have done for years.
It's just so much more convenient in every single respect.
It's always with you, you only need one free hand, it's got a built-in light, your phone is smaller and easier to carry than a book, it doesn't get beaten up or bent, you don't have to carry two books to work when you only have 50 pages of your current book left, I can buy and receive new books in seconds, and I can hoard unreasonable quantities of books without filling up my entire apartment (also makes moving house 10x easier...)
Ebook format would absolutely and comprehensively suck for textbooks - the UI is nowhere near good enough for thumbing through by feel, and no ereader screen has the size or resolution required for studying - but for ordinary fiction, there's just no downside except occasionally running out of battery.