I am really curious how people read on their phones, much less do so for extended periods of time. I can understand doing it while on the move for convenience, but actually preferring it to an iPad?
I know a co-worker who reads several novels a month (sometimes as much as 6 or 7) all on their iPhone X. I've asked them whether they've considered an iPad mini or Kindle since that would surely offer an appreciable difference in experience, but they claim it isn't necessary as the phone is "good enough."
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To answer the OP, my iPad is my most-used, least-essential, and most infuriating device. My iPhone is for on-the-go, music, and camera. My Mac is "home base" with servers, music management, file management, etc. My iPad is my real "personal computer." I use it to watch YouTube, Twitch, Hulu, etc. I browse the web, read books and manage, sometimes write, doodle, etc. Exactly the general content-consumption the most people use their computers for and, as a result, is my most-used device. Even so, it'd be the first to go if I had to make a choice because a Mac is 100% essential while the iPhone is what I use for core functionality.
I say the iPad is infuriating because I've never had a device that made me jump between all-in and not interest. Sometimes I feel like the iPad is really the best personal computer and I jump all in on using it all the time with all accessories. Then I get frustrated with the limitations or how I always feel like I should get the 11" iPad Pro (but when I use a smaller I want the 12.9"). I can't seem to find my perfect device. For 90% of my uses the Air would be fine, but I appreciate ProMotion and I want more than 256GB of storage so I'm stuck with the Pro. I have many aspirational uses for the iPad but usually just end up consuming content.