I have an iPhone and a Kindle. No iPad, but I do a lot of reading outdoors, so I don’t think it would work well for me because of the screen.
For me, the Kindle hasn’t replaced my books, but it has definitely supplemented them. If it’s a classic book, you can almost always find it somewhere free for Kindle. If it’s a heavy book I don’t want to lug around, I get it for my Kindle. (I loved it for using while nursing, because it’s so easy to hold one-handed! I read all of “Vanity Fair” while my youngest was a newborn … no way could I have read such a long book in paper form while nursing!) If there’s something I want to read NOW that I can’t get at the library or don’t want to wait for Amazon to deliver, then I can get it for my Kindle.
Most of what I have on it is classics, with some more current novels. I normally like to have reference-type books in paper form, so I can more easily grab them and look up what I want. But I do adore my Kindle and can’t imagine being without it! (Oh, and it’s great for reading stuff you don’t want to be seen reading, like the “Twilight” books, lol!)