Consider the alternatives.
HTML, too much re-rendering and re-formatting.
Word - Oh No. Not in a million lives. Have you seen the atrocity that is the "reading mode" in Word?
Epub / Mobi / Etc - I have never come across good readers.
For what its worth, PDFs are great for reading on larger screens like iPads. I read them on my mobile too, but that's not good for long reads.
The price is pretty reasonable up to about 7-8", and not too high to 10".
I splurged for 13" as I read a lot and often low-quality scans of small, three-column print.
The pixel density of an ebook reader (200-300 DPI) is far higher than even Retina displays. Monocrome/greyscale gives higher resolution as well (the three-elements-per-pel aspect of colour displays means you're always left with about 30% the effective resolution, though subpixel aliasing helps a lot).
Portrait will display a single page well (laptop displays suck for reading text), and for larger devices or larger-print materials, you can often manage a two-page-up display.
As a format, yes, what the fuck is everybody talking about? PDF is a disaster and should be killed off, HTML is great.